Laser system

ABSTRACT

A method and apparatus may comprise a line narrowed pulsed excimer or molecular fluorine gas discharge laser system which may comprise a seed laser oscillator producing an output comprising a laser output light beam of pulses which may comprise a first gas discharge excimer or molecular fluorine laser chamber; a line narrowing module within a first oscillator cavity; a laser amplification stage containing an amplifying gain medium in a second gas discharge excimer or molecular fluorine laser chamber receiving the output of the seed laser oscillator and amplifying the output of the seed laser oscillator to form a laser system output comprising a laser output light beam of pulses, which may comprise a ring power amplification stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-in-Part of U.S. patentapplication Ser. No. 11/787,180, filed on Apr. 13, 3007, entitled LASERSYSTEM, which was a Continuation-in-Part of U.S. patent application Ser.No. 11/584,792, filed on Oct. 20, 2006, entitled LASER SYSTEM, whichclaimed priority to U.S. Provisional Application Ser. No. 60/732,688,filed on Nov. 1, 2005, entitled 200 W GAS DISCHARGE EXCIMER OR MOLECULARFLUORINE MULTICHAMBER LASER, and to Ser. No. 60/814,293 filed on Jun.16, 2006, entitled 200 WATT DUV GAS DISCHARGE LASER SYSTEM, and to Ser.No. 60/814,424, filed on Jun. 16, 2006, entitled LONG LIVED MO IN MOPOCONFIGURED LASER SYSTEM, and was a Continuation-in-Part of U.S. patentapplication Ser. Nos. 11/521,904, filed on the Sep. 14, 2006, entitledLASER SYSTEM; and 11/522,052, filed on Sep. 14, 2006, entitled LASERSYSTEM; and 11/521,833, filed on Sep. 14, 2006, entitled LASER SYSTEM;and 11/521,860, filed on Sep. 14, 2006, entitled LASER SYSTEM; and11/521,834, filed on Sep. 14, 2006, entitled LASER SYSTEM; and11/521,906, filed on Sep. 14, 2006, entitled LASER SYSTEM; and11/521,858, filed on Sep. 14, 2006, entitled LASER SYSTEM; and11/521,835, filed on Sep. 14, 2006, entitled LASER SYSTEM; and11/521,905, entitled LASER SYSTEM, filed Sep. 14, 2006, the disclosuresof each of which are hereby incorporated by reference.

The present application is related to U.S. patent application Ser. No.11/447,380, entitled DEVICE AND METHOD TO STABILIZE BEAM SHAPE ANDSYMMETRY FOR HIGH ENERGY PULSED LASER APPLICATIONS, filed on Jun. 5,2006, and is related to U.S. patent application Ser. No. 10/881,533,entitled METHOD AND APPARATUS FOR GAS DISCHARGE LASER OUTPUT LIGHTCOHERENCY REDUCTION, filed on Jun. 29, 2004, and published on Dec. 29,2005, Pub. No. 20050286599, the disclosures of which are herebyincorporated by reference. The present application is also related toU.S. Pat. Nos. 6,549,551, issued on Apr. 15, 2003, to Ness et al,entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL; and6,567,450, issued on May 20, 2003, to Myers et al, entitled VERY NARROWBAND TWO CHAMBER HIGH REP RATE GAS DISCHARGE LASER SYSTEM; and6,625,191, entitled VERY NARROW BAND TWO CHAMBER HIGH REP RATE GASDISCHARGE LASER SYSTEM, issued on Sep. 23, 2003 to Knowles et al; and6,865,210, issued on Mar. 8, 2005, to Ershov et al, entitled TIMINGCONTROL FOR TWO CHAMBERED GAS DISCHARGE LASER SYSTEM; and 6,690,704,entitled CONTROL SYSTEM FOR TWO CHAMBER GAS DISCHARGE LASER SYSTEM,issued on Feb. 10, 2004 to Fallon et al; and 6,561,263, issued on May 6,2003, to Morton et al., entitled DISCHARGE LASER HAVING ELECTRODES WITHSPUTTER CAVITIES AND DISCHARGE PEAKS; and U.S. Pat. No. 6,928,093,entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, issued to Webb et al.on Aug. 9, 2005; the present application is also related to co-pendingU.S. patent application Ser. No. 10/781,251, filed on Feb. 18, 2004,entitled VERY HIGH ENERGY, HIGH STABILITY GAS DISCHARGE LASER SURFACETREATMENT SYSTEM; and Ser. No. 10/884,547, filed on Jul. 1, 2004,entitled LASER THIN FILM POLY-SILICON ANNEALING SYSTEM, published onJun. 30, 2005, Pub. No. US-2005-0141580; and Ser. No. 11/173,988,entitled ACTIVE BANDWIDTH CONTROL FOR A LASER, filed on Jun. 30, 2005;and to Ser. No. 11/169,203, entitled HIGH PULSE REPETITION RATE GASDISCHARGE LASER, filed on Jun. 27, 2005; and to Ser. No. 11/095,293,entitled GAS DISCHARGE LASER OUTPUT LIGHT BEAM PARAMETER CONTROL, filedon Mar. 31, 2005; and Ser. No. 11/095,976, entitled 6 KHZ AND ABOVE GASDISCHARGE LASER SYSTEM, filed on Mar. 31, 2005; and Ser. No. 11/201,877,filed on Aug. 11, 2005, entitled LASER THIN FILM POLY-SILICON ANNEALINGOPTICAL SYSTEM, Published on Dec. 8, 2005, Pub. No. US-2005-0269300; andSer. No. 11/254,282, entitled METHOD AND APPARATUS FOR GAS DISCHARGELASER BANDWIDTH AND CENTER WAVELENGTH CONTROL; and Ser. No. 11/346,519,filed on Feb. 1, 2006, entitled, VERY NARROW BAND, TWO CHAMBER, HIGH REPRATE GAS DISCHARGE LASER SYSTEM; and Ser. No. 11/323,604, filed on Dec.29, 2005, entitled MULTI-CHAMBER GAS DISCHARGE LASER BANDWIDTH CONTROLTHROUGH DISCHARGE TIMING; and Ser. No. 11/363,116, entitled VERY HIGHREPETITION RATE NARROW BAND GAS DISCHARGE LASER SYSTEM, filed on Feb.27, 2006; and Ser. No. 10/881,533, entitled METHOD AND APPARATUS FOR GASDISCHARGE LASER OUTPUT LIGHT COHERENCY REDUCTION, filed on Jun. 30,2004; and Ser. No. 10/847,799, entitled LASER OUTPUT LIGHT PULSESTRETCHER, filed on May 18, 2004; and U.S. patent application Ser. No.11/394,512, entitled CONFOCAL PULSE STRETCHER, filed on Mar. 31, 2006;the disclosures of each of which are incorporated herein by reference.

FIELD OF THE SUBJECT MATTER DISCLOSED

The subject matter disclosed is related to high power gas dischargelaser systems for DUV light sources, e.g., used in integrated circuitphotolithography, e.g., in highly line narrowed versions, e.g., forimmersion lithography and other lithography uses requiring high powerand/or requiring longer component life in lower power applications, orbroad band versions used for treatment of material on a workpiecesubstrate, e.g., laser annealing for low temperature poly-siliconprocessing (“LTPS”), such as thin beam sequential lateral solidification(“tbSLS”), and more particularly to a seed laser and amplification gainmedium system with an improved power amplification stage providinghigher gain and reduced ASE and coherency busting, e.g., for reductionin speckle.

BACKGROUND

Deep ultraviolet light sources, such as those used for integratedcircuit photolithography manufacturing processes have been almostexclusively the province of excimer gas discharge lasers, particularlyKrF excimer lasers at around 248 nm and followed by ArF lasers at 198 nmhaving been brought into production since the early 90's, with molecularfluorine F₂ lasers also having also been proposed at around 157 nm, butas yet not brought into production.

Immersion lithography, e.g., at 193 nm, by introducing water above thewafer allows for NA's up to 1.35 and this relaxes the k1 requirement,however, requiring higher power for exposures and potentially doublepatterning, which requires still more power delivered in the light fromthe laser light source.

Since the introduction of applicant's assignee's XLA-XXX, i.e., theXLA-100 initially, applicant's assignee's chosen solution fordelivering, e.g., much higher power than with earlier lasers, whilestill achieving various beam quality requirements, e.g., narrowerbandwidths as well as other light source requirements such as dosestability, has been a two chambered laser system comprising a seed laserpulse beam producing laser chamber, e.g., a master oscillator (“MO”),also of the gas discharge excimer variety, seeding another laser chamberwith an amplifying lasing medium, also of the same excimer gas dischargevariety, acting to amplify the seed beam, a power amplifier (“PA”).Other so-called master oscillator-power amplifier (“MOPA”) laser systemshad been known, mostly in the solid state laser art, essentially forboosting power output. Applicants' assignee came up with the concept ofthe utilization of seed laser chamber in which a seed laser wasproduced, with the view of optimizing that chamber operation forselecting/controlling desirable beam parameters, e.g., bandwidth, beamprofile, beam spatial intensity distribution, pulse temporal shape, etc.and then essentially amplifying the pulse with the desirable parametersin an amplifier medium, e.g., the PA. This breakthrough by applicants'assignee was able to meet the then current demands attendant to thecontinually shrinking node sizes for semiconductor photolithography DUVlight sources.

It is also possible to use an amplifying medium that comprises a poweroscillator. The PA of applicants' assignee is optimized both foramplification and for preservation of the desirable output beam pulseparameters produced in the MO with optimized, e.g., line narrowing. Anamplifier medium that is also an oscillator, a power oscillator (“PO”),has been proposed and used by applicants' assignee's competitorGigaPhoton, as evidenced in U.S. Pat. Nos. 6,721,344, entitled INJECTIONLOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued on Apr. 13, 2004 toNakao et al; 6,741,627, entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINELASER SYSTEM, issued on May 25, 2004 to Kitatochi et al, and 6,839,373,entitled ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4,2005 to Takehisha et al.

Unfortunately the use of an oscillator such as with front and rearreflecting mirrors (include a partially reflecting output coupler, andinput coupling, e.g., through an aperture in one of the or through,e.g., a 95% reflective rear reflector) has a number of drawbacks. Theinput coupling from the MO to the amplifier medium is very energyloss-prone. In the amplifier medium with such an oscillator cavityoptimized beam parameters selected, e.g., in the MO chamber, may bedenigrated in such an oscillator used as an amplifying medium. Anunacceptable level of ASE may be produced.

Applicant's propose an architecture that can preserve the optimized beamparameters developed in an MO chamber almost to the same degree asapplicants' assignee's present XLA XXX systems, while producing muchhigher output from the amplification medium or, alternatively givecurrent levels of output average power with strikingly reduced CoC forthe MO. Further, applicants believe that according to aspects ofembodiments of the subject matter disclosed, e.g., pulse-to-pulsestability can be greatly improved.

Buczek, et al, CO₂ Regenerative Ring Power Amplifiers, J. App. Phys.,Vol. 42, No. 8 (July 1971) relates to a unidirectional regenerative ringCO₂ laser with above stable (conditionally stable) operation anddiscusses the role of gain saturation on CO₂ laser performance. Nabors,et al, Injection locking of a 13-W Nd:YAG ring laser, Optics Ltrs., vol.14, No 21 (November 1989) relates to a lamp-pumped solid-state CW ringlaser injection locked by a diode-pumped solid state Nd:YAG masteroscillator. The seed is input coupled into the ring laser by a half-waveplate, a Faraday rotator and a thin film polarizer forming an opticaldiode between the seed laser and the amplifier. Pacala, et al., Awavelength scannable XeCl oscillator-ring amplifier laser system, App.Phys. Ltrs., Vol. 40, No. 1 (January 1982); relates to a single passexcimer (XeCl) laser system seeded by a line narrowed XeCl oscillator.U.S. Pat. No. 3,530,388, issued to Buerra, et al. on Sep. 22, 1970,entitled LIGHT AMPLIFIER SYSTEM, relates to an oscillator laser seedingtwo single pass ring lasers in series with beam splitter input couplingto each. U.S. Pat. No. 3,566,128, issued to Amaud on Feb. 23, 1971,entitled OPTICAL COMMUNICATION ARRANGEMENT UTILIZING A MULTIMODE OPTICALREGENERATIVE AMPLIFIER FOR PILOT FREQUENCY AMPLIFICATION, relates to anoptical communication system: with a ring amplifier. U.S. Pat. No.3,646,468, issued to Buczek, et al. on Feb. 29, 1972 relates to a lasersystem with a low power oscillator, a high power oscillator and aresonance adjustment means. U.S. Pat. No. 3,646,469, issued to Buczek,et al. on Feb. 29, 1097, entitled TRAVELLING WAVE REGENERATIVE LASERAMPLIFIER, relates to a laser system like that of the '468 Buczek patentwith a means for locking the resonant frequency of the amplifier tofrequency of the output of the oscillator. U.S. Pat. No. 3,969,685,issued to Chenausky on Jul. 13, 1976, entitled ENHANCED RADIATIONCOUPLING FROM UNSTABLE LASER RESONATORS relates to coupling energy froma gain medium in an unstable resonator to provide a large fraction ofthe energy in the central lobe of the far field. U.S. Pat. No.4,107,628, issued tot Hill, et al., on Aug. 15, 1978, entitled CWBRILLOUIN RING LASER, relates to a Brillouin scattering ring laser, withan acousto-optical element modulating the scattering frequency. U.S.Pat. No. 4,135,787, issued to McLafferty on Jan. 23, 1979, entitledUNSTABLE RING RESONATOR WITH CYLINDRICAL MIRRORS, relates to an unstablering resonator with intermediate spatial filters. U.S. Pat. No.4,229,106, issued to Domschner on Oct. 21, 1980, entitledELECTROMAGNETIC WAVE RING GENERATOR, relates to a ring laser resonatorwith a means to spatially rotate the electronic field distribution oflaser waves resonant therein, e.g., to enable the waves to resonate withopposite polarization. U.S. Pat. No. 4,239,341 issued to Carson on Dec.16, 1980, entitled UNSTABLE OPTICAL RESONATORS WITH TILTED SPHERICALMIRRORS, relates to the use of tilted spherical mirrors in an unstableresonator to achieve asymmetric magnification to get “simultaneousconfocality” and obviate the need for non-spherical mirrors. U.S. Pat.No. 4,247,831 issued to Lindop on Jan. 27, 1981, entitled RING LASERS,relates to a resonant cavity with at least 1 parallel sided isotropicrefracting devices, e.g., prisms, with parallel sides at an obliqueangle to part of light path that intersects said sides, along with ameans to apply oscillating translational motion to said refractingdevices. U.S. Pat. No. 4,268,800, issued to Johnston et al. on May 19,1981, entitled, VERTEX-MOUNTED TIPPING BREWSTER PLATE FOR A RING LASER,relates to a tipping Brewster plate to fine tune a ring laser locatedclose to a flat rear mirror A acting as one of the reflecting optics ofthe ring laser cavity. U.S. Pat. No. 4,499,582, entitled RING LASER,issued to Karning et al. on Feb. 5, 1980, relates to a ring laser systemwith a folded path pat two separate pairs of electrodes with a partiallyreflective input coupler at a given wavelength. U.S. Pat. No. 5,097,478,issued to Verdiel, et al. on Mar. 17, 1992, entitled RING CAVITY LASERDEVICE, relates to a ring cavity which uses a beam from a master laserto control or lock the operation of a slave laser located in the ringcavity. It uses a non-linear medium in the cavity to avoid the need ofinsulators, e.g., for stabilizing to suppress oscillations, e.g., asdiscussed in Col 4 lines 9-18. Nabekawa et al., 50-W average power,200-Hz repetition rate, 480-fs KrF excimer laser with gated gainamplification, CLEO (2001), p. 96, e.g., as discussed with respect toFIG. 1, relates to a multipass amplifier laser having a solid state seedthat is frequency multiplied to get to about 248 nm for KrF excimeramplification. U.S. Pat. No. 6,373,869, issued to Jacob on Apr. 16,2002, entitled SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION ATULTRAVIOLET WAVELENGTHS, relates to using an Nd:YAG source plus anoptical parametric oscillator and a frequency doubler and mixer toprovide the seed to a multipass KrF amplifier. U.S. Pat. No. 6,901,084,issued to Pask on May 31, 2005, entitled STABLE SOLID STATE RAMAN LASERAND A METHOD OF OPERATING SAME, relates to a solid-state laser systemwith a Raman scattering mechanism in the laser system oscillator cavityto frequency shift the output wavelength. U.S. Pat. No. 6,940,880,issued to Butterworth, et al. on Sep. 6, 2005, entitled OPTICALLY PUMPEDSEMICONDUCTOR LASER, relates to a optically pumped semiconductor laserresonance cavities forming part of a ring resonator, e.g., with a nonlinear crystal located in the ring, including, as discussed, e.g., withrespect to FIGS. 1, 2, 3, 5 & 6, having a bow-tie configuration. UnitedStates Published Patent Application No. 2004/0202220, published on Oct.14, 2004, with inventors Hua et al, entitled MASTER OSCILLATOR-POWERAMPLIFIER EXCIMER LASER SYSTEM, relates to an excimer laser system,e.g., with in a MOPA configuration, with a set of reflective optics toredirect at least a portion of the oscillator beam transmitted throughthe PA back thru PA ion the opposite direction. United States PublishedPatent Application No. 2005/0002425, published on Jan. 1, 2003, withinventors Govorkov et al, entitled MASTER-OSCILLATOR POWER-AMPLIFIER(MOPA) EXCIMER OR MOLECULAR FLUORINE LASER SYSTEM WITH LONG OPTICSLIFETIME, relates to, e.g., a MOPA with a pulse extender and using abeamsplitting prism in the pulse extender, a housing enclosing the(MO+PA) and reflective optics, with the pulse extender mounted thereon,and reflective optics forming a delay line around the PA. United StatesPublished application No. 2006/0007978, published on Jan. 12, 2006, withinventors Govokov, et al., entitled BANDWIDTH-LIMITED AND LONG PULSEMASTER OSCILLATOR POWER OSCILLATOR LASER SYSTEM, relates to a ringoscillator with a prism to restrict bandwidth within the oscillator.

U.S. Pat. No. 6,590,922 issued to Onkels et al. on Jul. 8, 2003,entitled INJECTION SEEDED F2 LASER WITH LINE SELECTION ANDDISCRIMINATION discloses reverse injection of and F₂ laser undesiredradiation centered around one wavelength through a single pass poweramplifier to selectively amplify a desired portion of the F₂ spectrumfor line selection of the desired portion of the F₂ spectrum in amolecular fluorine gas discharge laser. in F2 laser.

U.S. Pat. No. 6,904,073 issued to Yager, et al. on Jun. 7, 2005,entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS,discloses intracavity fluorine containing crystal optics exposed tolasing gas mixtures containing fluorine for protection of the optic.

Published International application WO 97/08792, published on Mar. 6,1997 discloses an amplifier with an intracavity optical system that hasan optical path that passes each pass of a sixteen pass through the sameintersection point at which is directed a pumping source to amplify thelight passing through the intersection point.

R. Paschotta, Regenerative amplifiers, found athttp://www.rp-photonics.com/regenerative_amplifiers.html (2006)discusses the fact that a regenerative amplifier, may be considered tobe an optical amplifier with a laser cavity in which pulses do a certainnumber of round trips, e.g., in order to achieve strong amplification ofshort optical pulses. Multiple passes through the gain medium, e.g., asolid state or gaseous lasing medium may be achieved, e.g., by placingthe gain medium in an optical cavity, together with an optical switch,e.g., an electro-optic modulator and/or a polarizer. The gain medium maybe pumped for some time, so that it accumulates some energy after which,an initial pulse may be injected into the cavity through a port which isopened for a short time (shorter than the round-trip time), e.g., withthe electro-optic (or sometimes acousto-optic) switch. Thereafter thepulse can undergo many (possibly hundreds) of cavity round trips, beingamplified to a high energy level, often referred to as oscillation. Theelectro-optic switch can then be used again to release the pulse fromthe cavity. Alternatively, the number of oscillations may be determinedby using a partially reflective output coupler that reflects someportion, e.g., around 10%-20% of the light generated in the cavity backinto the cavity until the amount of light generated by stimulatedemission in the lasing medium is such that a useful pulse of energypasses through the output coupler during each respective initiation andmaintenance of an excited medium, e.g., in a pulsed laser system. Uppalet al, Performance of a general asymmetric Nd:glass ring laser, AppliedOptics, Vol. 25, No. 1 (January 1986) discusses an Nd:glass ring laser.Fork, et al. Amplification of femtosecond optical pulses using a doubleconfocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989)discloses a seed laser/power amplifier system with multiple passesthrough a gain medium in a ring configuration, which Fork et al.indicates can be “converted into a closed regenerative multi passamplifier by small reorientations of two of the four mirrors thatcompose the resonator [and providing] additional means . . . forintroducing and extracting the pulse from the closed regenerator. Thisreference refers to the open-ended amplifier portion with fixed numberof passes through the amplifier portion (fixed by the optics an, e.g.,how long it takes for the beam to walk off of the lens and exit theamplifier portion as a “resonator”. As used herein the term resonatorand other related terms, e.g., cavity, oscillation, output coupler areused to refer, specifically to either a master oscillator or amplifierportion, the power oscillator, as lasing that occurs by oscillationwithin the cavity until sufficient pulse intensity exists for a usefulpulse to emerge from the partially reflective output coupler as a laseroutput pulse. This depends on the optical properties of the lasercavity, e.g., the size of the cavity and the reflectivity of the outputcoupler and not simply on the number of reflections that direct the seedlaser input through the gain medium a fixed number of times, e.g., a onepass, two pass, etc. power amplifier, or six or so times in theembodiment disclosed in Fork, et al. Mitsubishi published JapanesePatent Application Ser. No. JP11-025890, filed on Feb. 3, 1999,published on Aug. 11, 2000, Publication No. 2000223408, entitledSEMICONDUCTOR MANUFACTURING DEVICE, AND MANUFACTURING OF SEMICONDUCTORDEVICE, disclosed a solid state seed laser and an injection locked poweramplifier with a phase delay homogenizer, e.g., a grism or grism-likeoptic between the master oscillator and amplifier. United statesPublished application 20060171439, published on Aug. 3, 2006, entitledMASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, a divisional ofan earlier published application 20040202220, discloses as masteroscillator/power amplifier laser system with an optical delay pathintermediate the master oscillator and power amplifier which createsextended pulses from the input pulses with overlapping daughter pulses.

Partlo et al, Diffuser speckle model: application to multiple movingdiffusers, Appl. Opt. 32, 3009-3014 (1993), discusses aspects of specklereduction. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FORREDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. onAug. 3, 1993 discusses misaligned optical delay paths for coherencebusting on the output of gas discharge laser systems such as excimerlaser systems.

The power efficiency of a regenerative amplifier, e.g., using aswitching element, can be severely reduced by the effect of intracavitylosses (particularly in the electro-optic switch). Also, thereflectivity of a partially reflective output coupler can affect bothintracavity losses and the duration of the output pulse, etc. Thesensitivity to such losses can be particularly high in cases with lowgain, because this increases the number of required cavity round tripsto achieve a certain overall amplification factor. A possiblealternative to a regenerative amplifier is a multipass amplifier, suchas those used in applicants' assignee's XLA model laser systemsmentioned above, where multiple passes (with, e.g., a slightly differentpropagation direction on each pass) can be arranged with a set ofmirrors. This approach does not require a fast modulator, but becomescomplicated (and hard to align) if the number of passes through the gainmedium is high.

An output coupler is generally understood in the art to mean a partiallyreflective optic that provides feedback into the oscillation cavity ofthe laser and also passes energy out of the resonance cavity of thelaser.

In regard to the need for improvement of Cost Of Consumables, e.g., forArF excimer lasers, e.g., for photolithography light source use, KrF CoChas long been dominated by chamber lifetime, e.g., due to the robustnessof the optics at the higher 248 nm wavelength for KrF. Recent advancesin Cymer ArF optical components and designs have led to significantincreases in ArF optical lifetimes, e.g., ArF grating life improvementsdeveloped for the Cymer NL-7000A, Low intensity on LNMs, e.g., in twostage XLA systems. ArF etalon material improvements have contributed tolonger life for ArF wavemeters, stabilization modules, LAMs, SAMs, andBAMs. In addition KrF chamber lifetime has been significantly increasedwith Cymer ELS-7000 and ELS-7010 products, e.g., through the use ofproprietary electrode technology. However, longer life electrodetechnology requires specific operating parameters, such as are met inELS-7000 and ELS-7010 KrF chambers, XLA-200 and XLA-300 PA chambers.These parameters, however, are not able to be utilized, e.g., in any ofCymer's ArF XLA MO chambers because of the overall output powerrequirements of the system. Applicants propose ways to alleviate thisdetriment to cost of consumables in, e.g., the ArF dual chamber masteroscillator/amplifier products, used, e.g., for integrated circuitmanufacturing photolithography.

As used herein the term resonator and other related terms, e.g., cavity,oscillation, output coupler are used to refer, specifically to either amaster oscillator or amplifier portion, a power oscillator, as lasingthat occurs by oscillation within the cavity until sufficient pulseintensity exists for a useful pulse to emerge from the partiallyreflective output coupler as a laser output pulse. This depends on theoptical properties of the laser cavity, e.g., the size of the cavity andthe reflectivity of the output coupler and not simply on the number ofreflections that direct the seed laser input through the gain medium afixed number of times, e.g., a one pass, two pass, etc. power amplifier,or six or so times in the embodiment disclosed in Fork, et al. and noton the operation of some optical switch in the cavity. In some of theliterature an oscillator in which the round trip through theamplification gain medium, e.g., around a loop in a bow-tie or racetrackloop, is not an integer number of wavelengths, may be referred to as anamplifier, e.g., a power amplifier, while also constituting anoscillator laser. The term power amplification stage and morespecifically ring power amplification stage is intended herein to coverboth of these versions of a power oscillator, i.e., whether the paththrough the gain medium is an integer multiple of the laser systemnominal center wavelengths or not and whether the literature, or some ofit, would refer to such an “oscillator” as a power amplifier or not. Theclosed loop path or oscillation loop as used herein refers to the paththrough the amplification gain medium, e.g., an excimer or similar gasdischarge laser amplification stage, around which the seed laser pulselight oscillates in the amplification stage.

SUMMARY OF THE SUBJECT MATTER DISCLOSED

It will be understood by those skilled in that art that am method andapparatus are disclosed which may comprise a line narrowed or broadbandpulsed excimer or molecular fluorine gas discharge laser system whichmay comprise: a seed laser oscillator producing an output comprising alaser output light beam of pulses which may comprise a first gasdischarge excimer or molecular fluorine laser chamber; a line narrowingmodule within a first oscillator cavity; a laser amplification stagecontaining an amplifying gain medium in a second gas discharge excimeror molecular fluorine laser chamber receiving the output of the seedlaser oscillator and amplifying the output of the seed laser oscillatorto form a laser system output comprising a laser output light beam ofpulses, which may comprise: a ring power amplification stage which maycomprise: a partially reflecting optical element through which the seedlaser oscillator output light beam is injected into the ring poweramplification stage; a coherence busting mechanism intermediate the seedlaser oscillator and the ring power amplification stage comprising anoptical delay path having a delay length longer than the coherencelength of a pulse in the seed laser oscillator laser output light beamof pulses. The ring power amplification stage may comprise a bow-tieloop or a race track loop. The pulse energy of the output of the seedlaser oscillator being less than or equal to 1.0 mJ, 0.75 mJ, 0.5 mJ,0.2 mJ or 0.1 mJ. The output of the seed laser oscillator cavity may be≧5 mJ, 10 mJ or 15 mJ. The laser system may operating at an output pulserepetition rate of up to 12 kHz, ≧2 and ≦8 kHz, or ≧4 and ≦6 kHz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows as known MOPA configured multi-chamber laser system;

FIG. 2 shows aspects of an embodiment of the claimed subject matterdisclosed;

FIG. 3 shows aspects of an embodiment of the claimed subject matterdisclosed;

FIG. 4 shows aspects of an embodiment of the claimed subject matterdisclosed;

FIG. 5 shows aspects of an embodiment of the claimed subject matterdisclosed;

FIG. 6 shows aspects of an embodiment of the claimed subject matter isdisclosed;

FIG. 7 illustrates a timing and control regime according to aspects ofan embodiment of the subject matter disclosed;

FIG. 8 illustrates schematically multiple reflections within aoscillator cavity due only to optical misalignment, according to aspectsof an embodiment of the subject matter disclosed;

FIG. 9 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 10 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 11 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 12 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 13 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 14 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 15 illustrates schematically a top view of aspects of an embodimentof an input coupling mechanism useful according to aspects of anembodiment of the subject matter disclosed;

FIG. 16 illustrates schematically a side view of the input couplingmechanism of FIG. 15 useful according to aspects of an embodiment of thesubject matter disclosed;

FIG. 17 illustrates schematically input coupling useful according toaspects of an embodiment of the subject matter disclosed;

FIG. 18 illustrates schematically illustrates in cross section aspectsof an embodiment of an orthogonal injection seeding mechanism accordingto aspects of an embodiment of the subject matter disclosed;

FIG. 19 illustrates schematically illustrates in cross section aspectsof an embodiment of an orthogonal injection seeding mechanism accordingto aspects of an embodiment of the subject matter disclosed;

FIG. 20 illustrates schematically illustrates in cross section aspectsof an embodiment of a beam returner according to aspects of anembodiment of the subject matter disclosed;

FIG. 21 illustrates schematically illustrates in cross section aspectsof an embodiment of a beam returner according to aspects of anembodiment of the subject matter disclosed;

FIG. 22 illustrates schematically illustrates in cross section aspectsof an embodiment of a beam returner according to aspects of anembodiment of the subject matter disclosed;

FIG. 23 illustrates partly schematically in a partially cut awayperspective view an extension of a lasing chamber containing opticalelements in the chamber according to aspects of an embodiment of thesubject matter disclosed;

FIG. 24 illustrates measurements of forward and backward energy in aring power amplifier according to aspects of an embodiment of thesubject matter disclosed;

FIG. 25 illustrates measurements of forward and backward energy in aring power amplifier according to aspects of an embodiment of thesubject matter disclosed;

FIG. 26 illustrates schematically and in block diagram form a timing andcontrol system for a MOPO according to aspects of an embodiment of thesubject matter disclosed;

FIG. 27 illustrates the degree of saturation of a ring power oscillatorwith variation of MO output pulse energy according to aspects of anembodiment of the subject matter disclosed;

FIG. 28 shows schematically and in block diagram form a laser controlsystem according to aspects of an embodiment of the subject matterdisclosed;

FIG. 29 shows schematically and in block diagram form a laser controlsystem according to aspects of an embodiment of the subject matterdisclosed;

FIG. 30 shows schematically a seed injection mechanism and beam expanderaccording to aspects of an embodiment of the subject matter disclosed;

FIG. 31 shows schematically a coherency buster according to aspects ofan embodiment of the disclosed subject matter;

FIG. 32 shows schematically a coherence buster according to aspects ofam embodiment of the present invention;

FIG. 33 shows partly schematically and partly in block diagram for anexample of elements of a coherence busting scheme and the results ofaspects of the scheme according to aspects of an embodiment of thedisclosed subject matter;

FIG. 34 illustrates relative speckle intensity for a various E-Odeflector voltages related to relative timing between the E-O and thepulse generation in the seed laser according to aspects of an embodimentof the disclosed subject matter;

FIG. 35 illustrates pointing shift relative to E-O voltage according toaspects of an embodiment of the disclosed subject matter;

FIG. 36 illustrates an example of the timing of an E-O deflectionvoltage and a seed laser pulse spectrum according to aspects of anembodiment of the disclosed subject matter;

FIGS. 37A and B illustrate the effect on beam coherency from folding abeam upon itself according to aspects of an embodiment of the disclosedsubject matter;

FIG. 38 illustrates the effect of beam sweeping/painting on coherencyaccording to aspects of an embodiment of the disclosed subject matter;

FIG. 39 shows schematically and in cartoon fashion the effects ofmultiple coherence busing schemes;

FIG. 40 illustrates schematically a coherency reduction scheme accordingto aspects of an embodiment of the disclosed subject matter;

FIG. 41 illustrates results of simulated beam pulse flipping results;

FIG. 42 illustrates schematically and in partly block diagram form abeam combiner with divergence control according to aspects of anembodiment of the disclosed subject matter;

FIG. 43 illustrates a simulated E-O supply voltage with respect to aseed pulse intensity spectrum over time, according to aspects of anembodiment of the disclosed subject matter;

FIG. 44 illustrates a test E-O supply voltage with respect to a seedpulse intensity spectrum over time, according to aspects of anembodiment of the disclosed subject matter;

FIG. 45 illustrates a E-O cell drive circuit according to aspects of anembodiment of the disclosed subject matter;

FIG. 46 illustrates exemplary test results according to aspects of anembodiment of the disclosed subject matter;

FIG. 47 illustrates schematically and in block diagram form a broad bandlight source and laser surface treatment system using the DUV laserlight according to aspects of an embodiment of the disclosed subjectmatter;

FIG. 48 shows schematically a coherence buster optical delay pathaccording to aspects of an embodiment of the disclosed subject matter;

FIG. 49 shows schematically a coherence buster optical delay pathaccording to aspects of an embodiment of the disclosed subject matter.

DETAILED DESCRIPTION

According to aspects of an embodiment of the subject matter disclosed again amplification medium suitable for use with, e.g., an excimer ormolecular fluorine gas discharge seed oscillator laser in amulti-chamber (multi-portion) oscillator/amplifier configuration, thiscould be, e.g., a master oscillator power gain amplificationconfiguration may take advantage of a improved seed laser couplingarrangements, fundamentally designed to insert seed laser, e.g., masteroscillator seed output laser light pulse beam pulses, into an amplifyinggain medium, generally with little loss and with protection againstamplifier oscillation and/or ASE returning to the master oscillatorwhile the master oscillator laser medium is excited, such couldinterfere with the proper operation of the master oscillator, e.g., inconjunction with the line narrowing module producing the appropriatelynarrowed seed oscillator output laser light pulse beam pulse bandwidth.

According to aspects of an embodiment of the subject matter disclosed,however, a preferred configuration may comprise, e.g., a ring cavity,e.g., a power oscillator or Power Ring Oscillator (“PRO”). Such aconfiguration has been determined by applicants to be a very effectivesolution to going to higher power laser operation in a line narrowedmulti-portion (seed laser-amplifier) arrangement, particularly for gasdischarge seed laser to identical gas discharge amplifier lasermulti-portion laser systems. Such a laser system could be similar inoperation to applicants' employers' XLA series lasers, though with apower ring amplifier stage. Improvement in CoC may be attained accordingto a aspects of an embodiment of the subject matter disclosed. Also,however, a power ring amplification stage may be useful for otherapplications, including with seed lasers of other than the same type ofgas discharge laser, e.g., a solid state seed, e.g., matched to thelasing wavelength of an excimer or molecular fluorine amplifier, e.g.,by frequency shifting and/or frequency multiplication. In such systemspulse trimming may be useful for ultimate control of laser system outputlaser light pulse beam pulse parameters, e.g., bandwidth, bandwidthstability, output pulse energy, output pulse energy stability and thelike.

According to aspects of an embodiment of the subject matter disclosed aring cavity could use, e.g., such a beamsplitter, e.g., withP-polarization instead of the normally used S-polarization, sinceapplicants have found that the reflectivity of such an OPuS beamsplitterchanges from 60% for S-polarization to about 25% for P-polarization.

Thus, according to aspects of an embodiment of the subject matterdisclosed a ring cavity PO may be constructed, e.g., with a 24% outputcoupler, e.g., comprising an existing OPuS beamsplitter.

According to aspects of an embodiment of the subject matter disclosedapplicants propose to re-arrange, e.g., an existing XLA product, e.g.,with an excimer-based MO, from MOPA to MOPO (power oscillator),utilizing a power amplification stage, such as a ring poweramplification stage, in accordance with aspects of an embodiment of thesubject matter disclosed. Such a system can (1) improve energystability, e.g., by operating the amplification stage at saturation, oreffectively saturation, pulse to pulse, thereby more accurately insuringpulse to pulse energy stability, (2) achieve longer LNM life, e.g., byreducing the required MO output level to the μJ level, e.g., byincreasing the amplification in the amplification stage (overapplicants' assignees' traditional MOPA configuration) by approximatelyten fold, and (3) and exploit the ability to operate the MO at less than1 mJ in other ways beneficial to overall laser system operating life.

The advantages of the multi-chambered laser system according to aspectsof an embodiment of the subject matter disclosed enable meeting theabove discussed requirements for, e.g., higher power, better pulseenergy stability, better bandwidth control and lower achievablebandwidth, higher repetition rates and lower cost of operation. Forexample, the lower MO output energy requirement can, e.g., allow foreven better control of pulse parameters, e.g., bandwidth in the MO, withless energy loss in the MO, e.g., during line narrowing, and also lowerthe thermal impact, e.g., transients, and lower optical damage to theline narrowing optics, while maintaining or even increasing outputpower. Further according to aspects of an embodiment of the subjectmatter disclosed, e.g., through maintaining currently known MO pulseenergy output levels, very high (e.g., 10×) increases in currentlyavailable laser system output light average power may be attained. Thismay be beneficial both for line narrowed systems and for broad bandsystems, e.g., XeCl multi-chamber laser systems used, e.g., forannealing amorphous silicon on substrates, e.g., in LTPS processes,e.g., for the manufacture of crystallized substrates for the productionof, e.g., thin film transistors.

Turning now to FIG. 1 there is shown schematically and partly in blockdiagram form a more or less typical MOPA laser system 20, such asapplicants' assignees XLA multi-chamber MOPA laser systems. The lasersystem 20 may include, e.g., an oscillator seed laser chamber 22, and anamplifier gain medium laser chamber 24, e.g., a multi-pass poweramplifier (“PA”). The MO 22 may have associated with it, e.g., forapplications such as semiconductor manufacturing photolithography, aline narrowing module (“LNM”) 26, or if desired to be operated inbroadband mode, e.g., for application such as LTPS, it may not have linenarrowing. An output coupler 28, e.g., a partially reflective mirror,e.g., with a selected reflectivity for the applicable nominal centerwavelength, e.g., about 351 for XeF laser oscillators, 248 nm for KrFlaser oscillators, 193 nm for ArF laser oscillators and 157 formolecular fluorine laser oscillators, along with the rear end reflectionprovided by the LNM 26 (or a maximally reflective mirror for the givennominal center wavelength, not shown, substituted for the LNM 26 in thecase of broad band operation), may serve to form the laser 20 oscillatorcavity.

Relay optics 40, e.g., including a turning mirror 44 and a turningmirror 46, may serve to steer the seed oscillation laser 20 output laserlight pulse beam 62 pulses exiting a line center (center wavelength)analysis module (“LAM”) 42 along a light path (optical axis) 60 to theinput of the amplifier module lasing chamber 24. The LAM, in addition tocenter wavelength monitoring equipment (not shown) may include an MOenergy monitor 48, which may be provided with a small portion of thelaser output light pulse beam from the MO chamber 22 for metrologypurposes, e.g., for nominal center wavelength and energy detection, by abeam splitter 50 inside the LAM 42. The turning mirror 44 may providethe master oscillator 22 output laser light pulse beam 62 pulses to theturning mirror 46 along an optical path beam path, which may reflect thebeam 62 into the amplifier chamber 24 as a beam 64.

In the case of the system 20 of FIG. 1 the gain amplifier 24 is set upas a power amplifier, i.e., the light received from the MO, the MO seedoutput laser light pulse beam pulses passes through the gainamplification medium a fixed number of times, e.g., determined by theoptics, including the turning mirror 46, set up as illustratedschematically in FIG. 1 as an edge coupling optic and a beam returner(reverser) optic 70, e.g., a retro-reflecting mirror, discussed in moredetail below, along a beam path 72, exiting the laser gain mediumthrough a window 80, which may be, e.g., set at an angle, e.g., around70° to the exiting beam path 72 in order to optimize the transmissivityof the exiting light and the thermal loading for the given nominalcenter wavelength and the material of the window 80, e.g., CaF₂ forshorter wavelengths such as the nominal center wavelength for an ArFlaser system. The exit light 100 may also pass through a beam splitter75 within a bandwidth analysis module (“BAM”) 82. The laser systemoutput beam 100 may also pass through a first beam splitter 76 and asecond beam splitter 78 within a pulse stretcher 86, e.g., an OpticalPulse Stretcher (“OPuS”), such as are included with many of applicants'assignees' laser systems discussed above as a 4× pulse stretcher, e.g.,increasing the T_(is) of the laser system 20 output laser light pulsebeam 100 pulses exiting the system as beam 100 from about 17 ns to about40 ns and also from about 17 nm in length to about 40 nm in length, bydirecting the beam 100 into delay path 88, as is more fully describedin, e.g., U.S. Pat. No. 6,928,093, entitled LONG DELAY AND HIGH TISPULSE STRETCHER, issued to Webb et al. on Aug. 9, 2005 referenced above.

Also in the path of the laser system 20 output laser light pulse beam100 pulses may be, e.g., a beam expander 84, e.g., to decrease theenergy density on downstream optics, including the OPuS 86 beamsplitters 76, 78 and optical delay ling mirrors 90 and optics, e.g., inthe scanner (not shown) utilizing the laser system 20 output laser lightpulse beam 100 pulses. The laser system 20 may also include a shutter96, including, e.g., a shutter beam splitter 98, e.g., taking off aportion of the laser system 20 output laser light pulse beam 100 pulsesfor energy measurement in an output energy monitor (not shown) in theshutter 96.

OPuS 86

This existing XLA MOPA configuration, shown in FIG. 1 is illustratedschematically further in the sense that the illustration switchesbetween the horizontal and vertical axis in several places in order tomake this schematic illustration simpler and easier to understand. Noneof the concepts described here are impacted by properly drawing thehorizontal and vertical axis of the light paths.

Turning to FIG. 2 there is illustrated a conversion from a MOPAconfiguration to a master oscillator power oscillation amplificationstage configuration according to aspects of an embodiment of the subjectmatter disclosed, e.g., including a power ring amplification stage,e.g., with a ring cavity power oscillator (also known as a ring cavityregenerative amplifier) formed between the beam reverser 70 and thelower turning mirror 44, replaced with, e.g., an injection seedingmechanism 160 according to aspects of an embodiment of the subjectmatter disclosed. The BAM 82 as shown in FIG. 1 may be moved or itsfunctionality included within the shutter 96. According to aspects of anembodiment of the subject matter disclosed may include, e.g., placingthe a beam expander 142 comprising, e.g., first and second beamexpanding and dispersing prisms 146, 148 inside of an entrance windowand beam expander housing 140 which may be affixed to the gain mediumchamber 144 by a suitable means, e.g., welding or bolting with suitablesealing mechanisms. These optics 146, 148 may be placed inside the ringcavity formed between the orthogonal seed injection mechanism 160input/output coupling partially reflecting mirror 162 and the beamreturner 70, e.g., in order to reduce the energy density on the maximumreflector forming the beam returner 70, e.g., as illustrated in FIGS.20-22, which may be composed of, e.g., CaF₂, e.g., a beam splitter ofthe type currently used in applicants' assignee's OPuSs, and coated witha coating that, e.g., reflects 20% of the incident light, that makes upa portion 162 of the seed injection mechanism 160 of the ring poweramplification stage cavity discussed in more detail below. The beamreverser 70 may also be moved to inside the cavity gain medium cavitychamber 144 with the attachment of a housing 150 similar to that ofhousing 140.

Use of protective coatings on this optic 162 may be eliminated due,e.g., to the much lower energy output of the MO output laser light pulsebeam pulses according to aspects of an embodiment of the subject matterdisclosed. The beam expander optics 146, 148 and beam returner/reverser70, due to their makeup including a fluorine containing crystal andtheir exposure to fluorine in the lasing medium gas in the chamber 144and housings 140, 150 may be protected from optical damage. TheAMPLIFICATION STAGE chamber window 168 similarly constructed, e.g., of afluorine containing crystal, e.g., CaF₂, need not have a protectivecoating on its face exposed to the highest energy density, facing thering power oscillator oscillation cavity, due in part to its beamexpansion in the beam expanded 14Z in the cavity and also to usingaround a 45° angle, e.g., a 47 degree orientation.

According to aspects of an embodiment of the subject matter disclosed aring power oscillator cavity, e.g., as illustrated by way of example inFIG. 2, e.g., with beam expansion on the output coupler side of thechamber 144 and with the beam expansion prisms 146, 148 oriented toproduce a net dispersion, has a number of notable advantages, including,e.g., making very efficient use of seed energy, eliminating the need forprotective coatings for high power and very short nominal centerwavelength, especially at 193 nm and below, elimination of a rear windowon the laser chamber 144, dispersion in the cavity, which can, e.g.,help to decrease the ASE ratio, and acceptable energy density on theoptics, e.g., forming the output coupler portion 162 of the) seedinjection mechanism and the maximally reflective mirror (“Rmax”) portion164 of the seed injection mechanism, which may or may not be coated asneeded, e.g., as is done in applicants' assignee's OPuSs on existinglaser systems (beam splitters 76, 78 and mirrors 90). In addition, thearrangement can, e.g., perform the needed beam expansion function priorto the laser system output laser light pulse beam 100 entering the OPuS86, and the chamber 144 with additions 140 and 150 can easily be createdfrom applicant's assignee's existing chambers, e.g., XLA model chambers,e.g., by adding two “snouts” 140, 150, e.g., in place of existing windowmounting assemblies. This is shown, e.g., in more detail partiallyschematically FIG. in 23.

Further, all optics inside the chamber, e.g., including the snouts 140,150 can be, e.g., further removed from the source of chamber dust. Theconfiguration can also be made to fit, e.g., within a present XLA opticsbay.

As explained elsewhere, the ring cavity, e.g., as shown in FIGS. 12 and13, baffles may also be used with a relatively long round trip time,e.g., about 7 ns to traverse, e.g., from the input/output coupler beamsplitter 262 to the beam reverser 270 and back to the beam splitter 262.The ring power amplification stage can allow, e.g., the use of much lessenergy from the MO; approximately 100 uJ instead of the present value ofabout 1 mJ. This approximate 10 times or more reduction in MO energyoutput requirement according to aspects of an embodiment of the subjectmatter disclosed, can lead to, e.g., a 10 times increase in LNMlifetime, based on current LNM lifetime models utilized by applicants'assignee. Applicants' assignee has determined at least throughexperimentation and simulation that as little as 1-10 μJ can effectivelyseed the power ring amplification stage to produce upwards of 18 or somJ output from the amplification stage. Also, of course, especially inbroad band embodiments, but also in line narrowed embodiments, accordingto aspects of an embodiment of the subject matter disclosed using themore typical 1-3 mJ MO output greatly increases the average power out ofthe amplification stage over that of, e.g., current XLA-XXX lasersystems.

In addition, such small MO energy would likely allow use of a low MOchamber pressure with a number of longer chamber life benefits.

Rather than having to contemplate ways to simply survive high raw power,e.g., in the 200 W range, applicants, according to aspects of anembodiment of the subject matter disclosed contemplate being able toinstead focus on bettering energy stability, pointing stability, profilestability, and ASE stability of contemplated configuration whileoperating at full repetition rate, e.g., between 4 kHz and 6 kHz andeven above.

Turning now to FIG. 3 there is shown illustratively and partlyschematically and in block diagram form a ring power amplification stagelaser system 400, which in addition to the elements shown in FIG. 2 mayinclude, e.g., an injection seeding coupler mechanism 160 that isaligned with a chamber input/output window 94 and with beam expansionand dispersion optic 170, e.g., comprising beam expanding and dispersingprisms 172 and 174, which can, e.g., contract and steer and disperse thebeam 64 to one path 74 to the beam reverser 70 in chamber extension 150and back to the input/output coupler partially reflective mirror 162.Baffles 190 and 192, respectively may protect the optics in the rearsnout 150 and the front snout 140, form, e.g., dust circulating withinthe chamber 144.

In FIG. 4 a similar system A10, according to aspects of an embodiment ofthe subject matter disclosed, may include, e.g., the beamreturner/reverser 170 outside of the chamber 14A, which may incorporatemodified snouts 140, 150, e.g., to protect, respectively, frontinput/output window 194 and rear window 196 from circulating dust, alongwith baffles 190, 192. This embodiment may include, e.g., Brewsterwedges 420 and 430, for the purpose of, e.g., clearing the desirablepolarization by reflecting the undesirable polarization out of theoptical cavity.

FIG. 5 illustrates schematically and partly in block diagram format asystem 330 in which, e.g., a ring power amplification stage is set upby, e.g., the use of a polarizing beam splitter 336, half wave plate 338and output coupler 340. In operation the seed laser 334 feeds a seedlaser output laser light pulse beam 62 to the beam splitter 336 and thering cavity is set up with a single maximally reflective rear mirror243, e.g., and a partially reflective output couple 340.

Similarly a ring cavity may be set up according to aspects of anembodiment of the subject matter disclosed as illustrated in FIG. 6between an output coupler 410 and a rear cavity mirror 412.

There are a number of possible ways to couple the output laser lightpulse beam pulses from the MO to the power amplification stage, e.g., asillustrated schematically and partly in block diagram format in FIGS.9-11 and 14. As shown in FIG. 9, e.g., a partial reflection inputcoupled oscillator 200 may have a chamber 202 and, e.g., a partialreflective optic 204 as an input coupler to the oscillator chamber, witha front partially reflective optic output coupler 206. In operation, theMO output 62 enters the cavity, 200, shown to be a normal oscillator,rather than a ring oscillator, for purposes of, e.g., clarity ofdescription, and oscillates within the cavity formed by the entrancepartially reflecting optic 204 and the output coupler partiallyreflective optic 206, until oscillation results in a significant enoughpulse of laser system output light pulse beam 100 leaving the outputcoupler as is well understood in the art.

Illustrated schematically and partly in block diagram form in FIG. 10, apolarization input coupled oscillator 220 forming the ring poweramplification stage may include, e.g., a chamber 210, a polarizing beamsplitter 212, a quarter wave plate or Faraday rotator 214, a rearmaximally reflective mirror and an output coupler 216. In operation, themaximally reflective mirror 218 and the output coupler 216 for anoscillator cavity like that of FIG. 2, also shown as a regularoscillator, rather than a ring oscillator, for convenience. Thepolarizing beam splitter and quarter wave plate 214 serve, e.g., toisolate the MO from the amplification stage. The incoming beam 62 is ofa polarity that is reflected by the beam splitter 212 into the cavity,with, e.g., the quarter wave plate 214 transmitting the beam 62 into thecavity as circularly polarized and converting the return beam from theoutput coupler 216 to a polarization transmitted by the polarizing beamsplitter 212.

As illustrated in FIG. 11 a switched input/output coupler coupledoscillator 230 is shown schematically and partly in block diagram formatin which, e.g., a chamber 232 may be contained within a cavity formed,e.g., by a maximally reflective mirror 240 and a window 238, with, e.g.,an electro-optic switch, e.g., a Q-switch 236, acting as a switch toallow oscillation to build to a selected point and then the Faradayswitch is activated to allow a laser system output laser pulse beam 100pulse to be emitted.

Illustrated in FIG. 12 schematically and in partial block diagram formis a multi-pass regenerative ring oscillator laser system 250, accordingto aspects of an embodiment of the subject matter disclosed, which mayinclude, e.g., an amplifier chamber 252 and a seed laser 254 the system250 may also include an input/output coupler, e.g., an injection seedinput/output coupler mechanism 260. The input/output coupler 260 mayinclude, e.g., a partially reflective mirror 262, which may be, e.g., abeam splitter of the type used currently by applicants' assignee's OPuSssold with its laser equipment. The system 250 may also include, e.g., amaximally reflective optic 264, to steer the MO beam 62 into theelectrode region of the cavity as one pass beam 276 which may return tothe output coupler 262 as a second pass beam 278 from a beamreverser/returner 270, which may include, e.g., a first maximallyreflective mirror 272 and a second maximally reflective mirror 274. Insuch an embodiment where the beam(s) 276,278 each pass through the gainmedium formed between the electrodes (not shown) in each direction onceduring each round trip then the average energy output is greatlyincreased over a system in which only one direction of the “racetrack”arrangement passes through the gain medium.

FIG. 13 illustrates schematically and partly in block diagram form,according to aspects of an embodiment of the subject matter disclosed, amulti-pass regenerative ring power amplification stage laser system 280in the form of a bow-tie configuration, this may comprise, e.g., achamber 282, a seed laser 284, an injection seeding mechanism 260 and abeam reverser/returner 270, the latter two of which may be configured(angled), e.g., to effect a crossing of an input pass beam 286 and anoutput pass beam 288, e.g., at or near the intersection of therespective longitudinal and lateral center line axes of electrodes (notshown) generating a gain medium by gas discharge between the electrodes,and thus generally at the longitudinal and lateral axes intersection ofthe gain medium. In such an embodiment the angle between the two passesmay be almost imperceptively small so that, in effect the beams 286,288are almost aligned with the longitudinal center-line axis of thedischarge formed between the electrodes, one of which, e.g., beam 288,may also form the optical axis of the beam 100 of the laser system. Asshown schematically and well out of proportion in the applicable FIG.'sin this application neither beam path, e.g., 286,288 illustrated in FIG.13 may be shown to extend along the longitudinal centerline axis of theelectrodes or may be shown from a side view where the longitudinalcenterline axis is not discernible. In practice however, one pass wouldbe very slightly misaligned with their axis and the other essentiallyaligned with it insofar as such alignment is optically achievable andwithin the tolerances allowed for the optical trains of such lasersystems.

FIG. 15 illustrates schematically and partly in block diagram format aplan view of an embodiment 280 such as illustrated in FIG. 13. e.g.,where the MO output laser light pulse beam 62 comes from an MOpositioned above (in the direction perpendicular to the plane of thepaper) the power amplification stage chamber 282. Further, FIG. 16illustrates schematically and partly in block diagram format a side viewof the apparatus 280 of FIGS. 13 and 15. A turning mirror maximalreflector 430 turns the MO laser output light pulse beam 62 into, e.g.,an seed injection mechanism 260 and the crossed (bow-tie) passes 276,278 in the oscillation resonance cavity 424 formed by the rear mirrorbeam returner/reflector 270 (not shown in FIG. 16) and the injectionseed mechanism 270 input/output coupler partially reflective mirror 262to form laser system output light pulse beam 100 immerging through theoptic 262 acting as a usual oscillator cavity output coupler, as is wellknown in the art of gas discharge laser oscillator cavities with outputcouplers. As can be seen in FIGS. 15 and 16, the MO output beam 62enters the ring power amplification stage oscillation cavity through thepartially reflective mirror 262, which also forms the cavity outputcoupler from a direction, in relation to the axis of the system outputbeam 100 that prevents reverse coupling of the system output beam 100back to the MO. Also, part of the beam is transmitted to the maximallyreflective mirror 264 and the input/output coupler 262 forms part of themultiple pass path of the amplification stage partially reflecting partof the beam 278 to the mirror 264 on each round trip of the beams276,278.

FIG. 17 illustrates a schematically and partially in block diagramformat an input/output coupling scheme for a single rear mirror cavity300, e.g., with the multi-pass regenerative ring oscillator laser system300, according to aspects of an embodiment of the subject matterdisclosed, in the form of a bow-tie configuration, having, e.g., asingle maximally reflective rear cavity mirror 310. In operation the MOlaser input/output coupler, e.g., an orthogonal seed injection mechanism160 may direct the MO laser output light pulse beam 62 into the cavityformed at the rear by, e.g., a single maximally reflective mirror 310 toform, e.g., a “half” bow-tie configuration having a first pass path 76and a second pass path 78.

Turning now to FIG. 8 there is shown, according to aspects of anembodiment of the subject matter disclosed, what applicants refer to asan OPuS effect cavity 320 in which, e.g., a polarizing beam splitter 322and a maximally reflective rear cavity mirror 324 may be used along witha quarter wave plate 326 and an output coupler 328. The system 320,according to aspects of an embodiment of the subject matter disclosed,may, e.g., due to slight misalignment of optical elements 322 and 324,have multiple passes generated within the oscillator cavity formedbetween the rear mirror 324 and output coupler 328, caused, e.g., by themisalignment.

According to aspects of an embodiment of the subject matter disclosed,the orthogonal seed injection mechanism may comprise an orthogonalinjection seeding optic such as, e.g., optical element 350, illustratedin FIG. 18 schematically and in cross-section across the longitudinalextent of the optic 350. Optical element 350 may be made of, e.g., CaF₂,e.g., uncoated CaF₂, and may comprise, e.g., according to aspects of anembodiment of the subject matter disclosed, an external input/outputinterface facing 352, a total internal reflection face 354, and aninternal input/output interface face 356. In operation, as will beunderstood by those skilled in the optics art, the MO laser output lightpulse beam 62 may be incident upon received by the external input/outputinterface facing 352, e.g., at an incidence angle of, e.g., about 70°and be refracted within the optic 350 as beam 62′ to the total internalreflection face 354, which may be angled so as to totally internallyreflect the beam 62′ onto the internal input/output interface face 356as beam 62″ where it is refracted again upon entering the lasing gasmedium environment inside of the chamber (not shown in FIG. 19), e.g.,along a first path 76 and, after again passing through the gain medium,e.g., after reflection from a beam reverser (not shown in FIG. 19) itagain is incident on and transmits through the face 356, as beam 78 andis refracted within the optic 350 as beam 78′ to exit optic 350 asthrough this external input/output interface facing 352 the laser systemoutput light pulse beam 100. The beams 76, 78 according to aspects of anembodiment of the subject matter disclosed, along with the beam reverser(not shown in FIG. 19), may cross, e.g., as shown in FIG. 13 or not asshown in FIG. 12. It will be understood that the beam 78′ will alsopartially reflect onto the total internal reflecting surface 354 as beam78″ (62′) and the partially reflected portion will become beam 62″ andbeam 76 again, so that the optic 350 acts as an output coupler untilenough oscillations occur such that the stimulated emissions form asubstantial laser system output laser light pulse beam 100.

According to aspects of an embodiment of the subject matter disclosedanother version of a seed injection optic 360, illustrated in FIG. 19,schematically and in cross section across a longitudinal axis thereof,may comprise, e.g., an external input/output interface facing 362, atotal internal reflection face 364 and an internal input/outputinterface face 366. In operation the beam 62 from the MO may be incidentupon the external input/output interface facing 362 and be refractedwithin the optic 360 as beam 62′ to the total internal reflection face364, and be reflected as beam 62″ to the internal input/output interfaceface 366 where it will exit and again be refracted in the gas of thelasing medium environment as beam 76. After return from the beamreverser (not shown in FIG. 19) the beam 78 refracts in the optic 360 asbeam 78′ and transmits through the external input/output face 362 aslaser system output light pulse beam 100. It will also be understood bythose skilled in the laser art, as was so also with the embodiment ofFIG. 18, that the beam 78′ will also partially reflect onto the totalinternal reflecting surface 364 as beam 78″ (62′) and the partiallyreflected portion will become beam 62″ and beam 76 again, so that theoptic 360 acts as an output coupler until enough oscillations occur suchthat the stimulated emissions form a substantial laser system outputlaser light pulse beam 100.

According to aspects of an embodiment of the subject matter disclosed avariety of beam returners/reversers 370 may be utilized, e.g., asillustrated schematically in FIGS. 20-22. The optic 370 of FIG. 20 mayincorporate, e.g., an input/output face 372, a first total internalreflection face 374, a second total internal reflection face 376 and athird total internal reflection face 378, such that, in operation thebeam 76 transiting the gain medium in a first direction may be incidenton the face 372, and be reflected by the faces 374, 376 and 378 to exitthe optic 370 at the input/output interface face 372 as the beam 78passing through the gain medium in a second direction. A similar beamreverser 380 is illustrated in cross section and schematically in FIGS.21 and 22 wherein there are only two totally internal reflectingsurfaces 384, 386 and 394, 396 in the optics 380 and 390 respectively.

It will also be understood by those skilled in the optics art that withthree internal reflections, or with a three mirror arrangement, e.g., asis currently in use as a beam reverser on applicants' assignee's XLAmodel laser systems, the input beam 76 and output beam 78 will beeffectively aligned and parallel and that relationship does not change,e.g., with rotation of the optic, e.g., optic 370, e.g., about an axisperpendicular to the plane of the page of FIG. 20. For the reversers380, 390 respectively of FIGS. 21 and 22, the even number of internalreflections, e.g., two internal reflections allows for the beams 76, 78to have variable angular relationship in the plane of the paper of FIGS.21 and 22.

It will be understood by those skilled in the optics art that variouscombinations of the seed injection mechanism referred to in the presentapplication and beam reversers/returners may be utilized to get thebeams, e.g., on paths 76, 78 to cross, e.g., as illustrated in FIGS. 2,3, 4 and 13 or not, e.g., as illustrated in FIG. 12. Also, manipulatingthese optics will be understood to enable the selection of, e.g., acrossing point for the paths, e.g., 76, 78, e.g., with respect to theextent of the lasing gain medium, e.g., along the longitudinal and/orvertical axis of the lasing gas gain medium during a discharge betweenthe gain medium exciting electrodes. This may be utilized to varyparameters of the ultimate laser system output laser light pulse beampulses, e.g., energy, energy stability and the like.

Turning now to FIG. 23 there is illustrated a snout 140, variousversions of which are illustrated schematically, e.g., in FIGS. 2, 3 and4. The snout 140 may comprise, e.g., a window housing 550, which maycomprise an external mounting plate 552 and housing walls 554 machinedwith the housing plate 552 or otherwise affixed to the housing plate552. Also a window mounting plate 556 is shown. This window housing 550shown illustratively, according to aspects of an embodiment of thesubject matter disclosed, may be similar to such window housingscurrently being used on applicants' assignee's laser systems and mayhave a window housing end plate (not shown in this view for claritypurposes), similar to the laser end plate 570, such as is shown in FIG.23 at the laser side of the snout 140. The not shown window mountinglaser end plate may be at the point of the window mounting plate 556.Similarly the snout 140 may have a window end plate (not shown in thisFIG. for clarity) similar to the end plate 552 to affix the windowhousing 550 to the remainder of the snout 140. The laser chamber endmounting plate 568 may be attached to the laser chamber, e.g., 144 inFIG. 2, by mounting bolts 574, and have an aperture 572 through thebeam, e.g., 76 enters the chamber 144 and beam 78 returns from thechamber 144.

As shown in FIG. 23 partly schematically and partly in block diagramform the beam expander 142, e.g., comprising beam expanding prism 146and beam expanding prism 148, may be within the snout 140, as shownschematically in the cutaway of FIG. 23. At least one of the prisms 146,148 may be mounted on mounts (not shown) for movement one relative tothe other. This may be controlled, e.g., by a controller 600, e.g., byan actuator 580, e.g., a stepper motor or other suitable actuators knownin the art of laser system optic positioning control as referenced inone or more of the above referenced patents or co-pending applications,connected to the respective at least one prism, e.g., prism 148 by anactuator shaft 582, e.g. for rotation of the prism 148 on the axis ofthe shaft 582, to change its position relative to the prism 142 and alsoother optics, e.g., an orthogonal seed injection mechanism (not shown inFIG. 23) and/or the beam returner/reverser (not shown in FIG. 23).

Similarly as illustrated schematically and partly in block diagram form,e.g., in FIGS. 13 and 15, the beam reverser/returner 270 and/or the seedinput/output coupling optic 260 may be controlled by a controller 600controlling the operation of an actuator 590 for the beamreturner/reverser and 594 for the input output optic, e.g., anorthogonal seed injection mechanisms 260, with the actuators 590, 594connected to the controller 600 by control signal lines 592 and 596respectively.

A ring cavity, e.g., with an output coupler seed laser coupling, e.g.,an seed injection mechanism, while perhaps more complex a configuration,makes most efficient use of seed laser energy.

According to aspects of an embodiment of the subject matter disclosed,for the seed laser input/output coupling a range of maximally reflectingmirrors may be utilized, e.g., from about a square 45 degree Rmax toabout a square 30 degree Rmax, e.g., as is used on applicants'assignee's ArF 193 nm LNMs. Reflectivity for P-polarization is onlyabout 85% at 45 degrees. Instead of a perhaps more desirable 45 degreeoutput coupler with more desirable P-polarization and S-polarizationproperties, due to time constraints, applicants have so far onlyexamined the use of an OPuS beamsplitter, rotated to 45 degrees in thehorizontal, which provides, e.g., 24% reflectivity for P-polarizationand 60% reflectivity for S-polarization. A more desirable set of valuesmay be 20% for p-polarization and a smaller number, e.g., around 10% fors-polarization. In the development unit according to aspects of anembodiment of the subject matter disclosed a chamber window was held atthe standard 47 degree angle of incidence, rather than at Brewster'sangle, which could also be employed.

Applicants have measured P-polarization round trip transmission andS-polarization round trip transmission, as follows:

P-polarization S-polarization 0.24 0.60 0.85 0.85 (1.00)⁸ = 1.00 (0.85)⁸= 0.27 (0.969)⁸ = 0.78  (0.949)⁸ = 0.66  (1.00)² = 1.00 (0.85)² = 0.72 0.159  0.065with a ratio between S and P of 2.44:1.

According to aspects of an embodiment of the subject matter disclosedwhere attenuation of s-polarization may be needed, e.g., because of ASE,it may be achieved via Brewster reflections and insertion of partialreflectors in or into the power amplification stage cavity.

Also according to aspects of an embodiment of the subject matterdisclosed, pulse duration may be controlled, e.g., to control ASE, andif so this may be done, e.g., using optics and electronics, e.g., usefulin pulse trimming. Without controlling such timing and/or pulsetrimming, according to aspects of an embodiment of the subject matterdisclosed, e.g., the seed pulse duration may be longer than thatdesired, and, e.g., peak intensity may be lower for a given totalenergy.

MOPO energy vs. MO-AMPLIFICATION STAGE timing has been examined atdifferent values of seed laser energy, ArF chamber gas mixture,percentage reflectivity of output coupler (cavity Q) and seed laserpulse duration, with the results as explained in relation to FIG. 7.

ASE vs. MO-PO timing has been examined for different values of seedlaser energy, ArF chamber gas mixture, percentage reflectivity of outputcoupler (cavity Q) and seed laser pulse duration with the results alsoexplained in relation to FIG. 7.

The relationship between forward energy and seed energy has also beenexaminer and the results of which are illustrated, e.g., in FIGS. 24 and25. The measurements taken at what was considered to be optimum timingof the discharge in the lasing medium in the MO and the discharge in thelasing medium in the ring power amplification stage. In FIG. 24 thecurves 610 represent forward energy values and the curves 612representing backward energy values, with the square data pointsrepresenting operation with P90+P70 filters and the realignment wasperformed after insertion of partial reflectors with the results asshown in FIG. 25.

ASE can be of great concern with MOPO designs. Improper timing may leadto increased ASE up to and including generations of only ASE when the MOand power amplification stages are so mis-timed that essentially onlybroad band (ASE) lasing occurs in the power amplification stage, whichbeing an oscillator will lase when the discharge occurs between theelectrodes in the power amplification stage. Unlike a power amplifier,such as in applicants assignee's XLA-XXX laser system where the seedbeam passes through the amplification stage a fixed number of timesdepending on the optical arrangement, in systems according to aspects ofan embodiment of the subject matter disclosed amplified spontaneousemission (ASE) lasing occurs whether the seed laser pulse is present foramplification or not. Back scatter from the amplifier cavity optics canform a parasitic laser cavity. Some amplifier cavity optics can form anunintended laser cavity between the amplifier and MO. Therefore, carefulcontrol of timing is used, according to aspects of an embodiment of thesubject matter disclosed, to keep ASE below limits that reduces oreffectively eliminates the unwanted lasing. The back scattering orotherwise ASE at the MO may be measured by the ASE detector, which maycomprise, e.g., a fluorescence detector.

ASE measurements have been made with medium and small seed input energy.For example for medium energy, e.g., seed energy of around 50 μJ, with adischarge voltage Vco of around 950V, and with a AMPLIFICATION STAGE gasfill of 38/380, fluorine partial pressure/total pressure, it has beenshown that with a relative timing of between about −10 ns and +10 ns ofoptimum the ASE ratio is below about 3×10⁻⁵. With low seed energy, e.g.,around 5 μJ, with the same voltage and fill the ASE ratio is kept belowabout 6×10⁻⁴ between about 10 ns to +10 ns of relative timing.

While a ring cavity can produce very low ASE with 50 uJ of seed energy,the present implementation according to aspects of an embodiment of thesubject matter disclosed can reach the ASE upper limit specificationlimit at optimal relative timing of between about −10 ns and +10 ns ofoptimum with about 5 uJ of seed energy. Also according to aspects of anembodiment of the subject matter disclosed the seed pulse can be, e.g.,electro-optically trimmed to produce, e.g., a desired 10 ns pulseduration, even from an excimer seed laser with normally about a 30 nspulse duration. According to aspects of an embodiment of the subjectmatter disclosed applicants expect reduced ASE with 5 uJ of seed energy,e.g., due to higher peak intensity. Maintaining proper ASE performancemay require selecting proper amplifier cavity optics that haveappropriate selectivity to eliminate unwanted polarization (e.g.,utilizing appropriate coatings/angles of incidence, etc.), which canresult in better suppression of unwanted polarization, which can resultin reduced ASE, e.g., from the S-polarization. Creating dispersion inthe amplifier cavity, e.g., with beam expanding and dispersing prismshas also been determined by applicants, according to aspects of anembodiment of the subject matter disclosed, to be an effective methodfor further reducing the ASE ratio contributing to an effectively largeenough margin against whatever ASE specification is selected.

According to aspects of an embodiment of the subject matter disclosed amethod is proposed to reduce ASE in ring amplifiers, e.g., to takebetter advantage of other features of this architecture, e.g., low seedenergy, high efficiency, energy stability etc. Applicants propose tointroduce some broad band (at least much broader than the line narrowedseed radiation propagating in the opposite direction from that of themain radiation direction to increase ASE in this direction and reduceASE in the main direction. That is to say broad band gain will beutilized in the opposite path around the ring to reduce the availablegain for ASE in the main direction. This could be accomplished, e.g.,with some scatter of the seed laser beam from the optics, e.g., byfeeding florescence of the seed laser into the ring power amplificationstage. The broad band emission can thereby, e.g., deplete gain availableto the ASE and will be propagated oppositely to main radiationdirection, reducing broad band emission in the main direction.

According to aspects of an embodiment of the subject matter disclosed itwill be understood that solid state pulsed power systems such as themagnetically switched systems noted above in one or more of thereferenced patents or patent applications and as sold with applicants'assignee's laser systems, having very tightly controlled timing of thefirings of electric discharges between electrodes in the respective MOand amplification gain medium chambers, along with the properties of aring power amplification stage in a MOPRO configuration (e.g., operatingthe ring power amplification stage at or very near total saturation),enables the delivery to a lithography tool or an LTPS tool, or the like,laser system output light pulse beam pulses having about twice the dosestability as is currently achievable, e.g., in applicants' assignee'sXLA MOPA laser systems.

FIG. 26 shows schematically and in block diagram an energy/dose controlsystem 620 according to aspects of an embodiment of the subject matterdisclosed. As illustrated in block diagram form, the energy/dosecontroller 620 may include a solid state pulsed power system 624, suchas a magnetically switched pulsed power system as noted above, which maybe controlled, e.g., by a timing and energy control module 622 of thetype sold with applicants' assignee's current laser systems, e.g., XLAMOPA configured laser systems and discussed in one or more of the abovereferenced patents or co-pending applications. Such timing end energycontrol modules in combination with the SSPPM 624 are capable of veryfine pulse to pulse energy control, e.g., out of the MO and very fine,e.g., within a few nanoseconds, control of the relative timing of thefiring of the electric discharge between a first pair of electrodes (notshown) in the MO 24 and a second pair of electrodes, e.g., electrodes424 in the amplification stage 144 as illustrated, e.g., in FIGS. 15 and16. This enables, as discussed in one or more of the above referencedpatents and co-pending applications, e.g., the selection of a portion ofthe output laser light pulse beam pulse from the MO to initiate seedingof the amplification gain medium and the like, e.g., to controlbandwidth and also impact other overall laser system output light pulsebeam pulse parameters. This, in combination with the ring poweroscillator operating at or nearly at saturation, e.g., within about5-10% of saturation, or closer enables the system to delivery abouttwice the dose stability as is currently available in, e.g., lithographylaser light sources, such as applicants' assignee's XLA laser systems orexisting laser annealing light sources, e.g., for LTPS.

Referring to FIG. 31, a beam mixer 1050 is shown for operation on a beam1052 (which for illustrative purposes has been shown as having an upperwhite half and a lower black half). As explained in greater detailbelow, the beam mixer 1050 can be used to alter the intensity profile ofa beam, e.g. improving intensity symmetry along a selected axis of abeam, and can be used to reduce beam coherency, or both. For theembodiment shown, the beam mixer 1050 includes a beam splitter 1054 andmirrors 1056 a-c.

For the arrangement shown in FIG. 31, the beam 1052 can be initiallyincident upon the beam splitter 1054 whereupon a portion of the beam maydirected, via reflection, toward mirror 1056 a and the remainder istransmitted, (e.g., with substantially no change in direction) throughthe beam splitter 1054 and exits the beam mixer 1050 on an output beampath 1070. In one setup, a beam splitter 1054 reflecting about forty tosixty percent of the incident light, e.g., fifty percent, may be used.For this setup, about fifty percent of the initial beam incident uponthe beam splitter 1054 is directed toward the mirror 1056 a. For thebeam mixer 1050, mirrors 1056 a-c may typically be flat, maximumreflectivity mirrors. As shown in FIG. 31, mirror 1056 a may bepositioned and oriented to receive light from the beam splitter 1054 atan angle of incidence of approximately thirty degrees. As further shown,mirror 1056 b may be positioned and oriented to receive light reflectedfrom mirror 1056 a at an angle of incidence of approximately thirtydegrees, and mirror 1056 c may be positioned and oriented to receivelight reflected from mirror 1056 b at an angle of incidence ofapproximately thirty degrees.

Continuing with FIG. 31, light reflected from mirror 1056 c can be madeto be incident upon the beam splitter 1054 at an angle of incidence ofabout forty-five degrees. For a fifty percent reflectivity beamsplitter, about half of the light from mirror 1056 c is reflected ontothe output beam path 1070 and about half of the light from mirror 1056 cpasses through the beam splitter 1054 on a beam path toward mirror 1056a, as shown. Thus, the output beam path 1070 includes a combined beamcontaining the portion of the initial beam 1052 that passed through thebeam splitter 1054 and the portion of light from mirror 1056 c that isreflected from the beam splitter 1054. Similarly, the light on the pathfrom the beam splitter 1054 to mirror 1056 a includes a combined beamcontaining the portion of the initial beam 1052 that is reflected by thebeam splitter 1054 and the portion of light from mirror 1056 c that istransmitted through the beam splitter 1054.

The beam entering the beam mixer 1050 in FIG. 31 is shown illustrativelyas having a rectangular cross-section that defines a long axis 1058.This type of beam is typical of a laser beam produced by an excimerlaser with the long axis corresponding to the direction from onedischarge electrode to the other. A typical beam may have dimension ofabout 3 mm by 12 mm. Moreover, for the output of an excimer laser, theintensity profile in one axis, e.g., the long axis 1058 is typicallyunsymmetrical, whereas the intensity profile in the other axis, e.g.,the short axis (i.e. the axis normal to the long axis 1058) isapproximately Gaussian. Although the beam mixer 1050 shown isparticularly suitable for improving symmetry of a high power excimerdischarge laser, it is to be appreciated that it can be used inconjunction with other types of laser systems and for otherapplications, for example, the beam mixer may be used to reducecoherency in a beam generated by a solid state laser.

FIG. 31 shows that the beam extends along the axis 1058 from a firstedge 1060 to a second edge 1062. FIG. 31 also shows that the mirrors1056 a-c establishing a spatially inverting path which has a beginning1064 and an end 1066. As FIG. 31 illustrates, the inverting path may becharacterized in that a part of the beam near the first beam edge 1060at the beginning 1064 of the inverting path translates to the secondbeam edge at the end 1066 of the inverting path. More specifically, forthe mixer 1050 shown, a photon at the ‘top’ of the beam which strikesmirror 1056 a translates and leaves mirror 1056 c at the ‘bottom’ of thebeam. Since the inverting path constitutes a delay path, there will besome temporal stretching of the pulse, and this can be beneficial,especially in embodiments with the coherence busting mechanism betweenthe master oscillator/seed laser and the amplification gain medium,e.g., the ring power amplification stage. The pulse stretching betweenthe seed laser and amplification gain medium can stretch somewhat thepulse out of the amplification gain medium.

The beam mixer 1050 may be placed in between the seed beam laser portionand the amplifier laser portion of a MOPA or MOPRO (with, e.g., a ringpower amplification stage), configured multi-chambered laser system,such as that shown in FIGS. 1-6 and 9-16. Other forms of coherencybusting, of the passive type, may be used, e.g., between the MO and PAas discussed in above referenced in U.S. patent application Ser. No.11/447,380, entitled DEVICE AND METHOD TO STABILIZE BEAM SHAPE ANDSYMMETRY FOR HIGH ENERGY PULSED LASER APPLICATIONS, filed on Jun. 5,2006, and Ser. No. 10/881,533, entitled METHOD AND APPARATUS FOR GASDISCHARGE LASER OUTPUT LIGHT COHERENCY REDUCTION, filed on Jun. 29,2004, and published on Dec. 29, 2005, Pub. No. 20050286599, as well asSer. No. 11/521,904, entitled LASER SYSTEM, filed on the same day as thepresent application, referenced above.

It will be understood by those skilled in the art that as disclosed inthe present application according to aspects of an embodiment of thesubject matter disclosed, applicants have enabled the satisfaction ofcustomer demands, both from scanner makers and semiconductormanufacturer end users, that have been placed on light source suppliers,e.g., ArF light sources, beyond even the traditionally expected powerand bandwidth improvements. For example, further CoC Improvement isdemanded because, e.g., ArF is now used in high volume production, e.g.,on cost sensitive products, the industry expectation of equivalentreductions in cost of operation and thus cost of consumables for ArF aswas historically demanded in KrF as that technology matured. Inaddition, energy stability improvements are met by the subject matterdisclosed, e.g., critical dimension variation sensitivity to dose, whichhas become greater with the advent of low K1 lithography techniques. Thedouble exposure concept, e.g., also trades off between overlay and dosecontrol. Optical maskless lithography will require single pulse exposurecontrol, improved by aspects of embodiments of the subject matterdisclosed.

According to aspects of an embodiment of the subject matter disclosedCost Of Consumables Improvement, e.g., in XLA MO Chamber Life areenabled, e.g., because the parameters that lead to long chamber life didnot produce sufficient MO energy for proper operation in the XLA MOPAconfiguration, e.g., a requirement of at least about a 1 mJ MO outputenergy. According to aspects of an embodiment of the subject matterdisclosed significantly less than 1 mJ energy is required from the MOcavity, e.g., for lithography uses, thus allowing significant reductionsin CoC for such low light sources. Therefore, e.g., with operatingparameters more conducive to long electrode life, e.g., the system mayproduce, e.g., around 100 μJ from the MO cavity. This can, e.g., in ArFlasers, e.g., increase the MO chamber life time to at or about that ofthe PA chamber lifetime, an approximately a 10× increase in MO lifetimebefore replacement as a consumable while still attaining system outputaverage power from the amplification stage for effective lithographywith 60-100 watts or so output. MO cavity optics, LNM optics and the MOoutput coupler, according to aspects of an embodiment of the subjectmatter disclosed, experience lower 193 nm intensity, ensuring very muchlonger optics lifetimes.

With regard to energy stability improvements, the Cymer XLA light sourceled to a significant improvement in energy stability by exploiting thesaturation effects in the PA of a MOPA configuration, e.g., with a twopass PA amplification. The slope of Eout vs. Ein for XLA is about ⅓. MOenergy instabilities are reduced by a factor of 3× when passed throughsuch a PA. However, even with the 3× improvement through the PA, theMOPA system energy stability is still greatly impacted by, e.g., the MOenergy instability. MO and PA contributions are about equal. Othercontributions such as, voltage regulation, timing jitter, and MOpointing jitter are relatively smaller contributors, but notinsignificant. The PA energy stability performance falls somewherebetween a typical broadband oscillator and a fully saturated amplifier.

According to aspects of an embodiment of the subject matter disclosed, arecirculating ring configuration, e.g., a power ring amplificationstage, operates in a much stronger region of saturation. The slope ofEout vs. Ein for a seed laser/amplification gain medium system, e.g.,with a ring power amplification stage has been measured by applicants'employer at 0.059. MO energy instabilities can be reduced by a factor of17×, e.g., when passed through a recirculating ring oscillator, e.g., apower ring amplification stage. At the planned operating point of about100 μJ of MO energy, the Eout vs. Ein slope has been observed at orsimulated at 1/17.

With the recirculating ring configuration the amplification stage energystability will exhibit the characteristics of a fully saturatedamplifier. Applicants expect at a minimum to see about a 1.5-2×improvement in energy stability.

$\sigma_{System} = \sqrt{{\frac{1}{17}\sigma_{MO}^{2}} + \sigma_{PA}^{2} + \sigma_{voltage}^{2} + \sigma_{timing}^{2} + \sigma_{{MO}\mspace{14mu}{Pointing}}^{2}}$

A larger improvement is expected in the MO σ², e.g., due to reducedthermal transient effects at the approximately 10× lower required MOoutput pulse energy, a large improvement is expected in theamplification gain medium σ², e.g., due to operating at or nearsaturation. Improvement is also expected for MO pointing σ².

Optics may be utilized to create, e.g., a combined bow-tie andrace-track oscillation path for four passes per oscillation path, oroptics may be utilized to, e.g., create two or more overlapping bow-tiesor race tracks. Bow tie arrangements may sometimes be referred to in theart as a cat's cradle. Characteristic of such amplifier media, e.g.,regenerative or recirculating ring power amplification stage can includeparallel planarity which could be a stable oscillator, e.g., in halfplanes or an unstable oscillator. The beam returner/beam reverser couldutilize multiple mirrors or prisms or a combination thereof, positionedinside or outside the chamber or a combination thereof, e.g., dependantupon exposure to certain levels of energy density by one or more of theoptical elements. Unwanted light, e.g., mostly ASE is discriminatedagainst in a variety of ways, e.g., preferentially being created in adirection opposite from the regeneration path of the oscillations of theseed laser pulse beam, e.g., in the ring power amplification stage.Coated optics may be avoided not just in the amplifier gain mediumcavity but also, e.g., for the output coupler or in the LNM of the MO,e.g., for the nominal center wavelength selection maximally reflectingmirror (Rmax) or on the grating. Expanding the beam within the amplifierstage cavity, e.g., corresponding to the vertical direction of aBrewster angle window, also can serve to protect optical elements in thering power oscillator cavity as well as disperse the light to lessenASE. The output coupler portion of the seed inject mechanism may, e.g.,have a reflectivity of around 20% for the desired (in-band) frequencies(or polarization or both) and 100% for undesired light, e.g., ASE, as afurther means of ASE reduction. The output coupler may have, e.g., bothinput and output side surfaces coated with a 100% reflectivity coatingfor the unwanted light wavelengths, e.g., for ASE which may also, e.g.,clean up birefringence induce polarization. Beam expansion may also beable to be performed with multiple prisms, some one or more of which maybe inside and/or outside of the chamber enclosure. That is, while one ormore of such prisms may be inside the chamber enclosure and exposed tothe fluorine containing laser gas mixture, at least one may also beoutside the chamber.

Turning now to FIG. 7 there is shown a chart illustrating by way ofexample a timing and control algorithm according to aspects of anembodiment of the subject matter disclosed. The chart plots laser systemoutput energy as a function of the differential timing of the dischargein the seed laser chamber and the amplification stage, e.g., the ringpower amplification stage as curve 600, which is referred to herein asdtMOPO for convenience, recognizing that the amplification stage in someconfigurations may not strictly speaking be a PO but rather a PA thoughthere is oscillation as opposed to the fixed number of passes through again medium in what applicants' assignee has traditionally referred toas a power amplifier, i.e., a PA in applicants' assignee's MOPA XLA-XXXmodel laser systems, due, e.g., to the ring path length's relation tothe integer multiples of the nominal wavelengths. Also illustrated is arepresentative curve of the ASE generated in the amplification stage ofthe laser system as a function of dtMOPO, as curve 602. In addition,there is shown an illustrative curve 604 representing the change in thebandwidth of the output of the laser system as a function of dtMOPO.Also illustrated is a selected limit for ASE shown as curve 606.

It will be understood that one can select an operating point on the ASEcurve at or around the minimum extremum and operate there, e.g., bydithering the control selection of dtMOPA to, e.g., determine the pointon the operating curve 602 at which the system is operating. It can beseen that there is quite a bit of leeway to operate around the minimumextremum of the ASE curve 602 while maintaining output pulse energy onthe relatively flat top portion of the energy curve to, e.g., maintainlaser system output pulse energy and energy σ, and the related dose anddose σ constant, within acceptable tolerances. In addition as shown,there can be a concurrent use of dtMOPO to select bandwidth from a rangeof bandwidths while not interfering with the E control just noted.

This can be accomplished regardless of the nature of the seed laserbeing used, i.e., a solid-state seed or a gas discharge laser seed lasersystem. Where using a solid-state seed laser, however, one of a varietyof techniques may be available to select (control) the bandwidth of theseed laser, e.g., by controlling, e.g., the degree of solid-state seedlaser pumping. Such pump power control may, e.g., put the pumping powerat above the lasing threshold in order to select a bandwidth. Thisselection of bandwidth may shift or change the pertinent values of thecurve 604, but the laser system will still be amenable to the type of Eand BW control noted above using dtMOPO to select both a BW andconcurrently an operating point that maintains the output energy of thelaser system pulses at a stable and more or less constant value in theflat top region of the illustrated energy curve 600. It is also possibleto use a non-CW solid state seed laser and to adjust the outputbandwidth. For example, selection of the output coupler reflectivity ofthe master oscillator cavity (cavity-Q) can adjust the output bandwidthof the seed laser system. Pulse trimming of the seed laser pulse mayalso be utilized to control the overall output bandwidth of the lasersystem.

It can be seen from FIG. 7 that either the selected ASE upper limit orthe extent of the portion of the energy curve that remains relativelyflat with changes in dtMOPO may limit the range of available bandwidthfor selection. The slope and position of the BW curve also can be seento influence the available operating points on the ASE curve to maintainboth a constant energy output and a minimum ASE while also selectingbandwidth from within an available range of bandwidths by use of theselection of a dtMOPO operating value.

It is similarly known that the pulse duration of discharge pulses in agas discharge seed laser, among other things, e.g., wavefront controlmay be used to select a nominal bandwidth out of the seed laser and thusalso influence the slope and/or position of the BW curve 604 asillustrated by way of example in FIG. 7.

Turning now to FIG. 28, there is shown an example in schematic and blockdiagram form of a laser system controller 620, according to aspects ofan embodiment of the subject matter disclosed. The controller 620 maycomprise part of a laser system including, e.g., a seed laser 622, suchas a gas discharge laser known in the art of the type XeCl, XeF, KrF,ArF or F₂ or the like, which may have associated with it a linenarrowing module 624, as is known in the art, for selecting a particularnominal center wavelength and at the same time narrowing the bandwidthto the ranges discussed above in the present application. The seed laser622 may produce a seed laser output beam 626, which may pass through abeam splitter 630 which diverts a small portion of the output beam 626to a metrology unit metrology module 632, which may include, among otherthings, an MO energy detector and a wavemeter, measuring, e.g., centerwavelength and bandwidth.

The output beam 626 may then be turned by a maximally reflective mirror634 (for the nominal center wavelength) to a seed injection mechanism636. The seed injection mechanism may include, e.g., a partiallyreflective optical element 638 and a maximally reflective opticalelement 640, and may be two separate elements or a single optic asdiscussed elsewhere in the present application. As discussed elsewhere,the seed injection mechanism may inject the seed laser output pulse beam626 into an amplification gain medium, such as a ring poweramplification stage 650 along an injection path 652, whereby the pulsebeam oscillates in a loop also comprising a return reverse path 654 andthe partially reflective input/output coupler 638 until such time asenough amplification in the ring power amplification stage occurs bylaser light oscillation in the cavity for the input/output coupler 638to pass a laser system output light pulse beam 658 on to a tool usingthe output light. A beam splitter 654 can divert a small portion of theoutput beam 658 into a metrology unit 656 which may measure, e.g.,output energy and bandwidth. A metrology unit 642 connected directly tothe amplification gain medium laser 650 which can measure, e.g., ASE inthe laser chamber 650.

A controller 660, which may comprise a processor 662, receives inputsfrom the various metrology units 632, 642 and 656, and others asappropriate, and utilize them as part of control algorithms referencedin one or more of the above noted patents and co-pending applicationsand also incorporate the control algorithm noted above regardingoperating at or around the ASE curve minimum while maintaining energyconstant and also selecting bandwidth within the limits imposed by aselected ASE limit. In addition, as is shown in one or more of the abovereferenced patents and co-pending applications the controller 660 mayalso control the timing of the creation of an output pulse in the seedlaser and the creation of the output pulse in the amplification gainmedium (dtMOPO for short) and also provide control signals to the linenarrowing module, e.g., to control bandwidth, e.g., by wavefrontmanipulation or optical surface manipulation as discussed above and inone or more of the above referenced patents and co-pending patentapplications.

Turning now to FIG. 29 there is illustrated schematically and in blockdiagram form a laser system 680 like that of the laser system 620 ofFIG. 28 with the exception that the seed laser 682 is, e.g., a solidstate seed laser with an associated frequency converter 684 to, e.g.,modify the wavelength of the output of the seed laser 682 to awavelength suitable for amplification in the amplification gain mediumstage 650. In addition, the controller 660 may provide inputs to theseed laser 682 to control both the timing of the creation of the seedlaser pulse and the bandwidth, e.g., by modifying the pumping power, asdiscussed above.

According to aspects of an embodiment of the subject matter disclosedone may need to select an edge optic, that is an optic that may have tobe used, and thus perhaps coated, all the way to its edge, which can bedifficult. Such an optic could be required, e.g., between the outputcoupler, e.g., 162 shown in FIG. 2 and the maximum reflector, e.g., 164,shown in FIG. 2, together forming a version of a seed injectionmechanism 160, shown in FIG. 2, e.g., depending upon the separationbetween the two, since there may be too little room to avoid using anedge optic. If so, then the edge optic should be selected to be theRmax, e.g., because of the ray path of the exiting beam as it passesthrough the OC portion 162. From a coatings standpoint it would bepreferable to have the OC be the edge optic because it has fewer layers.However, an alternative design, according to aspects of an embodiment ofthe subject matter disclosed has been chose by applicants and isillustrated schematically and by way of example in FIG. 30, e.g.,wherein the use of an edge optic can be avoided, e.g., if a large enoughspacing is provided between out-going and in-coming ring poweramplification stage beams, e.g., as created by the beam expander, 142shown in FIG. 2, e.g., prisms 146, 148. For example, about a 5 mmspacing between the two beams has been determined to be satisfactoryenough to, e.g., to avoid the use of any edge optics.

As illustrated by way of example in FIG. 30 the laser system, e.g.,system 110 illustrated by way of example in FIG. 2, may produce a lasersystem output pulse beam 100. e.g., using a ring power amplificationstage 144 to amplify the output beam 62 of a master oscillator 22 in aring power amplification stage 144. A beam expander/disperser 142, shownin more detail by way of an example of aspects of an embodiment of thesubject matter disclosed may be comprised of a firstexpansion/dispersion prism 146 a, and a second expansion/dispersionprism 146 b, and a third prism 148.

The seed injection mechanism 160 may comprise a partially reflectiveinput/output coupler 162, and a maximally reflective (Rmax) mirror 164,illustrated by way of example and partly schematically in FIG. 30 in aplan view, i.e., looking down on the seed injection mechanism and mexpansion/dispersion 160 and the ring power amplification stage chamber(not shown) into and out of which, respectively the beams 74 and 72traverse, that is from the perspective of the axis of the output beam 62traveling from the master oscillator chamber 22, which in such anembodiment as being described may be positioned above the chamber 144(the beam 62 having been folded into the generally horizontallongitudinal axis as shown (the beam also having been expanded in theMOPuS in its short axis, as described elsewhere, to make it generally asquare in cross-sectional shape.

With regard to the configuration of the beam expansion prisms 146 a, 146b and 148 inside the ring power amplification stage cavity a similararrangement may be provided to that of the beam expansion on the outputof the power amplifier (“PA”) stage in applicants' assignee's XLA-XXXmodel laser systems, e.g., with a 4× expansion, e.g., provided by a68.6° incident and 28.1° exit, e.g. on a single prism or on two prismswith the same incident and exit angles. This can serve to, e.g., balanceand minimize the total Fresnel losses. Reflectivity coatings, e.g.,anti-reflectivity coatings may be avoided on these surfaces since theywill experience the highest energy densities in the system. According toaspects of an embodiment of the subject matter disclosed the beamexpander/disperser 160 may be implemented with the first prism 146 splitinto to small prisms 146 a, and 146 b, which may be, e.g., 33 mm beamexpander prisms, e.g., truncated, as shown by way of example in FIG. 30,to fit in the place where one similarly angled prism could fit, with thesplit prism having a number of advantages, e.g., lower cost and theability to better align and/or steer the beams 72, 74 (in combinationwith the beam reverser (not shown in FIG. 30) and the system output beam100.

The master oscillator seed beam 62 may enter the seed injectionmechanism 160 through the beam splitter partially reflective opticalelement 162, acting as an input/output coupler, to the Rmax 164 as beam62 a, from which it is reflected as beam 74 a to the first beam expanderprism 146 a, which serves to de-magnify the beam in the horizontal axisby about ½× (it remains about 10-11 mm in the vertical axis into theplane of the paper as shown in FIG. 30). The beam 74 b is then directedto the second beam expansion prism 148, e.g., a 40 mm beam expansionprism, where it is again de-magnified by about ½× so the totalde-magnification is about ¼× to form the beam 74 entering the gainmedium of the ring power amplification stage (not shown in FIG. 30. thebeam is reversed by the beam reverser, e.g., a beam reverser of the typecurrently used in applicants' assignee's XLA-XXX model laser system PAsand returns as beam 72 to the prism 148, e.g., having crossed in thegain medium in a bow-tie arrangement or having traveled roughlyparallel, perhaps overlapping to some degree in a version of arace-track arrangement. From prism 148 where the beam 72 is expanded byroughly 2× the beam 72 b is directed to prism 142 b and is expanded afurther approximately 2× into beam 72 a. Beam 72 a is partiallyreflected back to the Rmax as part of beam 62 a and is partiallytransmitted as output beam 100, which gradually increases in energyuntil an output beam pulse of sufficient energy is obtained by lasingoscillation in the ring power amplification stage. The narrowing of thebeam entering the amplification gain medium, e.g., the ring poweramplification stage has several advantageous results, e.g., confiningthe horizontal widths of the beam to about the width of the electricalgas discharge between the electrodes in the gain medium (for a bow-tiearrangement the displacement angle between the two beams is so smallthat they each essentially stay within the discharge width of a few mmeven thought they are each about 2-3 mm in horizontal width and for therace track embodiment, the bean 72 or the bean 72 only passes throughthe gain medium on each round trip, or the beams may be furthernarrowed, or the discharge widened, so that both beams 72,74 passthrough the discharge gain medium in each round trip of the seed beams72, 74.

The positioning and alignment of the prisms 146 a, 146 b and 148,especially 146 a and 146 b can be utilized to insure proper alignment ofthe output beam 100 from the ring power amplification stage into thelaser output light optical train towards the shutter. The beam leavingthe input/output coupler 162 may be fixed in size, e.g., in thehorizontal direction, e.g., by a horizontal size selection aperture 130,forming a portion of the system aperture (in the horizontal axis) toabout 10.5 mm. Another aperture, e.g., in the position roughly of thepresent PA WEB, e.g., in applicants' assignee's XLA-XXX laser systemproducts, can size the beam in the vertical dimension. Since the beamhas about a 1 mRad divergence, the sizing may be slightly smaller ineach dimension than the actual beam dimensions wanted at the shutter,e.g., by about 1 mm. According to aspects of an embodiment of thesubject matter disclosed applicants propose that a system limitingaperture be positioned just after the main system output OPuS, e.g., a4× OPus. A ring power amplification stage aperture may be located about500 mm further inside the laser system. This distance is too great toavoid pointing changes turning into position changes at the specifiedmeasurement plane (present system aperture). Instead the limiting systemaperture can be located just after the OPuS, and may have a 193 nmreflecting dielectric coating instead of a stainless steel platecommonly used. This design can allow for easier optical alignment, whileat the same time reduce heating of this aperture.

According to aspects of an embodiment of the subject matter disclosed,applicants propose to implement a relatively stress-free chamber windowarrangement similar to or the same as that discussed in the abovereferenced co-pending U.S. patent application, e.g., at least on thebean reverser side of the chamber, because of the use of, e.g., a PCCFcoated window a this location.

According to aspects of an embodiment of the subject matter disclosed,applicants propose to, e.g., place ASE detection, e.g., backwardpropagation ASE detection, in either the LAM or in an MO wavefrontengineering box (“WEB”), or in a so-called MOPuS, which can, e.g.,include elements of the MOWEB from applicants' assignee's existingXLA-XXX model laser systems along with the mini-OPuSs discussedelsewhere in this application and in the co-pending application11/521,833 referenced herein, as well as, e.g., beam expansion, e.g.,using one or more beam expansion prisms to expand the output beam of theMO in its short axis, e.g., to form generally a square cross-sectionalbeam. The current MO WEB and its beam turning function is representedschematically as the turning mirror, e.g., 44 shown in FIG. 2. As apreference, however, the backward propagation detector may be placed“in” the MO WEB/MOPuS, that is, e.g., by employing a folding mirror(fold #2), e.g., 44 in FIG. 2, with, e.g., a reflectivity of R=95%instead of R=100% and monitoring the leakage through this mirror 44.Some drift and inaccuracy of this reading may be tolerated, e.g., sinceit may be utilized as a trip sensor (i.e. measurements in the vicinityof 0.001 mJ when conditions are acceptable—essentially no reverse ASE—asopposed to around 10 mJ when not acceptable—there is reverse ASE), e.g.,when the ring power amplifier is not timed to amplify the seed pulse,but still creates broad band laser light. Existing controller, e.g., TEMcontroller, cabling and ports and the like for new detectors may beemployed. The detector may, e.g., be the detector currently used byapplicants' assignee on existing XLA-XXX model laser systems to measurebeam intensity, e.g., at the laser system output shutter.

According to aspects of an embodiment of the disclosed subject matterone or more mini-OPuS(s), which may be confocal, such that they arehighly tolerant to misalignment and thus of potentially low aberration,e.g., for the off-axis rays needed in the proposed short OPuS(s), theso-called mini-OPuS, can have delay times of 4 ns and 5 ns respectively,where more than one is employed. These values were chosen so that bothOPuSs exhibit low wavefront distortion with spherical optics in additionto appropriate delay paths for coherence busting. The low wavefrontrequirement may actually prevent significant speckle reduction from themini-OPuS(s) unless an angular fan-out from the output of themini-OPuS(s) is generated, e.g., by replacing a flat/flat compensatingplate with a slightly wedged plate, so that the transmitted beam and thedelayed beam in the mini-OPuS are slightly angularly offset from eachother. The laser beam, e.g., from the master oscillator is partiallycoherent, which leads to speckle in the beam. Angularly offsetting thereflected beam(s) reentering the mini-OPuS output with the transmittedbeam, along with the delay path separation of the main pulse into themain pulse and daughter pulses, can achieve very significant specklereduction, e.g., at the wafer or at the annealing workpiece, arisingfrom the reduction in the coherence of the laser light source pulseilluminating the workpiece (wafer or crystallization panel). This can beachieved, e.g., by intentionally misaligning the delay path mirrors,probably not possible with a confocal arrangement, but also with theaddition of a slight wedge in the delay path prior to the beam splitterreflecting part of the delayed beam into the output with the transmittedbeam and its parent pulse and preceding daughter pulses, if any. Forexample, a 1 milliradian wedge in the plate will produce an angularoffset in the reflected daughter pulse beam of 0.86 milliradians.

The optical delay path(s) of the mini-OPuS(s) may have other beneficialresults in terms of laser performance and efficiency. According toaspects of an embodiment of the disclosed subject matter, as illustratedschematically in FIG. 48, the laser beam, e.g., seed beam 500 from theseed source laser (not shown in FIG. 48, may be split into two beams502, 504 using a partially reflective mirror (beam splitter) 510. Thismirror 510 transmits a percentage of the beam into the main beam 502 andreflects the rest of the beam 500 as beam 504 into an optical delay path506. The part 502 that is transmitted continues into the rest of thelaser system (not shown in FIG. 48). The part 504 that is reflected isdirected along a delay path 506 including, e.g., mirrors 512, 514 and516, with mirror 514 being displaced perpendicularly to the plane of thepaper in the schematic illustration, in order to allow the main beam 502to reenter the rest of the laser system, e.g., to form a laser outputbeam or for amplification in a subsequent amplification stage. The beam504 may then be recombined with the transmitted portion 502 of theoriginal beam 500. The delayed beam 504 may be passed through a wedge(compensator plate) 520 essentially perpendicularly arranged in the pathof beam 504. Thus, the daughter pulse beam(s) 504 from the delay path506 are slightly angularly displaced from the main part of the beam inthe transmitted portion 502 in the far field. The displacement may be,e.g., between about 50 and 500 μRad.

The length of the delay path 506 will delay the beam pulses so thatthere is a slight temporal shift between the part of the beam that istransmitted and the part that is reflected, e.g., more than thecoherence length, but much less than the pulse length, e.g., about 1-5ns. By selecting the appropriate path length, which determines the delaytime, the addition of the two beams can be such that the energy in thepulse is spread into a slightly longer T_(is), which in combination withlater pulse stretching in the main OPuS(s) can improve laserperformance, as well as providing other beneficial laser performancebenefits.

Two mini-OPuSs may be needed to achieve the desired effect. The offsettime between the pulses from the two mini-OPuss may be, e.g., onenanosecond. Based upon optical and mechanical considerations, the delaysselected for the stretchers may be, e.g., a 3 ns delay path in the firstmini-OPus and a 4 ns delay path in the second. If the delay is shorter,the optical system, e.g., if it uses confocal or spherical mirrors canintroduce unacceptable aberrations. If the delay is longer, it may bedifficult to fit the system into the available space in the lasercabinet. The distance the beam must travel to achieve the 3 ns delay is900 mm and to delay by 4 ns is 1200 mm. A confocal optical system 520,minimizing the sensitivity to misalignment, illustrated schematically inFIG. 49 may consist of two mirrors 522, 524, whose focal points arelocated at the same position in space and whose center of curvatures arelocated at the opposite mirror, along with a beam splitter 526. Acompensator plate 530 (e.g., a wedge) can be added to insure that thereflected beam and the transmitted beam are slightly misaligned as notedabove with respect to FIG. 48. In this case, the compensator plate isplaced in the path of the delayed beam at an angle for properfunctioning.

The delay path time(s) in the mini-OPuS(s) for coherence busting andother purposes may be as short as about the temporal coherence lengthand as long as practical due to the noted optical and spaceconsiderations, such as misalignment and aberration tolerance. If thereare two or more mini-OPuSs then the delay path in each must be differentin length, e.g., by more than the coherence length and selected suchthat there is no significant coherence reaction (increase) due to theinteraction of daughter pulses from the separate OPuS(s). For examplethe delay path times could be separated by at least a coherence lengthand by not more than some amount, e.g., four or five coherence lengths,depending on the optical arrangement.

According to aspects of an embodiment of the subject matter disclosedapplicants propose to employ a coherence-busting optical structure that,e.g., generates multiple sub-pulses delayed sequentially from a singleinput pulse, wherein also each sub-pulse is delayed from the followingsub-pulse by more than the coherence length of the light, and inaddition with the pointing of each sub-pulse intentionally chirped by anamount less than the divergence of the input pulse. In additionapplicants propose to utilize a pair of coherence-busting optical delaystructures, where the optical delay time difference between the pair ofoptical delay structures is more than the coherence length of the inputlight. Each of the two optical delay structures may also generatesub-pulses with controlled chirped pointing as noted in regard to theaspects of the previously described coherence busting optical delaystructure.

According to aspects of an embodiment of the disclosed subject mattertwo imaging mini-OPuSs, which may be confocal, such that they are highlytolerant to misalignment and thus of potentially low aberration, e.g.,for the off-axis rays needed in the proposed short OPuSs, the so-calledmini-OPuSs, and can have delay times of 4 ns and 5 ns respectively.These values were chosen so that both OPuSs exhibit low wavefrontdistortion with spherical optics. The low wavefront requirement mayprevent significant speckle reduction from the mini-OPuSs unless anangular fan-out from the mini-OPuSs is generated, e.g., by replacing aflat/flat compensating plate with the slightly wedged plate.

It will be understood by those skilled in the art that according toaspects of an embodiment of the disclosed subject matter, adequatecoherence busting may be achieved sufficiently to significantly reducethe effects of speckle on the treatment of a workpiece being exposed toillumination from the laser system, such as in integrated circuitphotolithography photoresist exposure (including the impact on line edgeroughness and line width roughness) or laser heating, e.g., for laserannealing of amorphous silicon on a glass substrate for low temperaturerecrystallization processes. This may be accomplished by, e.g., passingthe laser beam, either from a single chamber laser system or from theoutput of a multi-chamber laser system or from the seed laser in such amulti-chamber laser system before amplification in another chamber ofthe multi-chamber laser system, through an optical arrangement thatsplits the output beam into pulses and daughter pulses and recombinesthe pulses and daughter pulses into a single beam with the pulses anddaughter pulses angularly displaced from each other by a slight amount,e.g., between, e.g., about 50 μRad and 500 μRad and with each of thedaughter pulses having been delayed from the main pulse(s), e.g., by atleast the temporal coherence length and preferably more than thetemporal coherence length.

This may be done in an optical beam delay path having a beam splitter totransmit a main beam and inject a portion of the beam into a delay pathand then recombining the main beam with the delayed beam. In therecombination, the two beams, main and delayed, may be very slightlyangularly offset from each other (pointed differently) in the far field,referred to herein as imparting a pointing chirp. The delay path may beselected to be longer than the temporal coherence length of the pulses.

The angular displacement may be accomplished using a wedge in theoptical delay path prior to the delayed beam returning to the beamsplitter which wedge imparts a slightly different pointing to thedelayed beam (a pointing chirp). The amount of pointing chirp, as notedabove may be, e.g., between about 50 and 500 μRad.

The optical delay paths may comprise two delay paths in series, eachwith a respective beam splitter. In such an event each delay path can bedifferent in length such that there is not created a coherence effectbetween the main and daughter pulses from the respective delay paths Forexample, if the delay in the first delay path is 1 ns the delay in thesecond delay path could be about 3 ns and if the delay in the firstdelay path is 3 ns the delay in the second could be about 4 ns.

The wedges in the two separate delay paths may be arranged generallyorthogonally to each other with respect to the beam profile, such thatthe wedge in the first delay path can serve to reduce coherence(speckle) in one axis and the wedge in the other delay path can reducecoherence (speckle) in the other axis, generally orthogonal to thefirst. Thus, the impact on speckle, e.g., contribution to line edgeroughness (“LER”) and/or line width roughness (“LWR”), e.g., at thewafer in exposure of photoresist in an integrated circuit manufacturingprocess can be reduced along feature dimensions in two different axes onthe wafer.

According to aspects of an embodiment of the subject matter disclosed,with, e.g., a 6 mRad cross of the bowtie in a bowtie ring poweramplification stage, the magnification prisms inside the ring cavity maybe slightly different for the in-going and outgoing beams, and could bearranged so that the beam grows slightly as it travels around the ringor shrinks slightly as it travels around the ring. Alternatively, andpreferably according to aspects of an embodiment of the subject matterdisclosed, a result of breaking the larger beam expansion prism into twoseparate pieces, e.g., enabled by larger spacing between out-going andin-coming beams, e.g., about 5-6 mm, as illustrated by way of example inFIG. 30, applicants propose to adjust the angles of the two prisms,e.g., 146, 148 shown schematically in FIG. 4, such that they result inthe same magnification for both out-going and in-coming beams, e.g.,beams 100 and 62, respectively, shown illustratively and schematicallyin FIG. 30.

According to aspects of an embodiment of the subject matter disclosedapplicants propose to place the Rmax, e.g., 164 and the OC, e.g., 162portions of the version of the seed injection mechanism containing anRmax 164 and an OC 162, e.g., along with the positioning of the systemhorizontal axis beam output aperture on that same stage. This enables,e.g., prior alignment of each as an entire unit and removes the need forfield alignment of the individual components. This can allow, e.g., forthe position of the Rmax/OC assembly, e.g., 160, shown in FIG. 2 (a seedinjection mechanism) to be fixed, just like the OC location in aapplicants' assignee's single chamber oscillator systems (e.g., XLS 7000model laser systems) is fixed. Similarly, such an arrangement can allowfor the achievement of tolerances such that the Rmax/OC are positionedrelative to the system aperture properly without need for significantongoing adjustment. The beam expansion prism may be moveable foralignment of the injection seed mechanism assembly with the chamber 144of the amplification gain medium and the output beam 100 path with thelaser system optical axis.

According to aspects of an embodiment of the subject matter disclosedapplicants propose to position a mechanical shutter to block the MOoutput from entering the ring, when appropriate, similar to such as areutilized on applicants' assignee's OPuSs, e.g., to block them duringalignment and diagnosis. The exact location could be, e.g., just abovethe last folding mirror prior to the ring power amplification stage,where the mini-OPuSes are protected during unseeded ring poweramplification stage alignment and operation.

It will be understood by those skilled in the art that there isdisclosed herein an apparatus and a method for use of a line narrowedpulsed excimer or molecular fluorine gas discharge laser system whichmay comprise a seed laser oscillator producing an output comprising alaser output light beam of pulses comprising: a first gas dischargeexcimer or molecular fluorine laser chamber; a line narrowing modulewithin a first oscillator cavity; a laser amplification stage containingan amplifying gain medium in a second gas discharge excimer or molecularfluorine laser chamber receiving the output of the seed laser oscillatorand amplifying the output of the seed laser oscillator to form a lasersystem output comprising a laser output light beam of pulses, which maycomprise a ring power amplification stage. The ring power amplificationstage may comprise an injection mechanism which may comprise a partiallyreflecting optical element, e.g., a beam splitter, which may be apartially reflective optical element and may be polarization sensitive,through which the seed laser oscillator output light beam is injectedinto the ring power amplification stage. The seed laser oscillator mayprovide an output at less than 1 mJ, and may be as low as, e.g., around1-10 μJ. The ring power amplification stage may comprise a bow-tie loopor a race track loop. The pulse energy of the output of the seed laseroscillator may be less than or equal to 0.1 mJ, or 0.2 mJ, or 0.5 mJ, or0.75 mJ. The ring power amplification stage may amplify the output ofthe seed laser oscillator cavity to a pulse energy of over 1 mJ, or 2mJ, or 5 mJ, or 10 mJ or 15 mJ. The laser system may operate at, e.g.,an output pulse repetition rate of up to 12 kHz, or ≧2 and ≦8 kHz or ≧4and ≦6 kHz. The laser system may comprise a seed laser oscillatorproducing an output comprising a laser output light beam of pulses whichmay comprise a first gas discharge excimer or molecular fluorine laserchamber; a laser amplification stage containing an amplifying gainmedium in a second gas discharge excimer or molecular fluorine laserchamber receiving the output of the seed laser oscillator and amplifyingthe output of the seed laser oscillator to form a laser system outputcomprising a laser output light beam of pulses, which may comprise aring power amplification stage. The laser system may operating within amatrix of operating values that can serve to optimize laser lifetime andproduce other advantageous results including better pulse energystability and the like, e.g., the seed laser oscillator containing alasing gas comprising a mixture of fluorine and other gases andoperating at ≦350 kPa of total lasing gas pressure, or ≦300 kPa of totallasing gas pressure, or ≦250 kPa of total lasing gas pressure, or ≦200kPa of total lasing gas pressure or ≧35 kPa of fluorine partialpressure, or ≧30 kPa of fluorine partial pressure, ≧25 kPa of fluorinepartial pressure, or ≧20 kPa of fluorine partial pressure andcombinations of the above. The system may further comprise a coherencebusting mechanism intermediate the seed laser oscillator and the ringpower amplification stage. The coherence busting mechanism maysufficiently destroy the coherence of the output of the seed laserreduce speckle effects in a processing tool using the light from thelaser system. The coherence busting mechanism may comprise a first axiscoherence busing mechanism and a second axis coherence busing mechanism.The coherence busting mechanism may comprise a beam sweeping mechanism.The beam sweeping mechanism may be driven in one axis by a first timevarying actuation signal. The beam sweeping mechanism may be driven inanother axis by a second time varying actuation signal. The firstactuation signal may comprise a ramp signal and the second actuationsignal may comprise a sinusoid. The time varying signal(s) may have afrequency such that at least one full cycle occurs within the timeduration of a seed laser output pulse. The coherence busting mechanismmay comprise an optical delay path with misaligned optics producing ahall of mirrors effect. The coherence busting mechanism may comprise anoptical delay path longer than the coherence length of the seed laseroutput pulse. The coherence busting mechanism may comprise an activeoptical coherency busting mechanism and a passive optical coherencybusting mechanism. The active coherence busting mechanism may comprise abeam sweeping device and the passive coherence busting mechanism maycomprise an optical delay path. The coherence busting mechanism maycomprise a first optical delay path with a delay longer than thecoherence length of the seed laser output pulse and a second opticaldelay path in series with the first optical delay path and having adelay longer than the coherence length of the seed laser output pulse.The delay of the second optical delay path may be greater than or equalto about 3 times the coherence length of the seed laser output pulse.The coherence busting mechanism may comprise a pulse stretcher. Thepulse stretcher may comprise a negative imaging optical delay path. Thepulse stretcher may comprise a six mirror OPuS. The coherence bustingmechanism may a beam flipping mechanism. The system and method maycomprise the use of a line narrowed pulsed excimer or molecular fluorinegas discharge laser system which may comprise a seed laser oscillatorproducing an output comprising a laser output light beam of pulses whichmay comprise a first gas discharge excimer or molecular fluorine laserchamber; a line narrowing module within a first oscillator cavity; alaser amplification stage containing an amplifying gain medium in asecond gas discharge excimer or molecular fluorine laser chamberreceiving the output of the seed laser oscillator and amplifying theoutput of the seed laser oscillator to form a laser system outputcomprising a laser output light beam of pulses, which may comprise aring power amplification stage; a coherence busting mechanismintermediate the seed laser oscillator and the ring power amplificationstage. The ring power amplification stage may comprise an injectionmechanism comprising a partially reflecting optical element throughwhich the seed laser oscillator output light beam is injected into thering power amplification stage. The system and method may comprise theuse of a broad band pulsed excimer or molecular fluorine gas dischargelaser system which may comprise a seed laser oscillator producing anoutput comprising a laser output light beam of pulses which may comprisea first gas discharge excimer or molecular fluorine laser chamber; alaser amplification stage containing an amplifying gain medium in asecond gas discharge excimer or molecular fluorine laser chamberreceiving the output of the seed laser oscillator and amplifying theoutput of the seed laser oscillator to form a laser system outputcomprising a laser output light beam of pulses, which may comprise aring power amplification stage; a coherence busting mechanismintermediate the seed laser oscillator and the ring power amplificationstage. The ring power amplification stage may comprise an injectionmechanism comprising a partially reflecting optical element throughwhich the seed laser oscillator output light beam is injected into thering power amplification stage. The system and method may comprise theuse of a pulsed excimer or molecular fluorine gas discharge laser systemwhich may comprise a seed laser oscillator producing an outputcomprising a laser output light beam of pulses which may comprise afirst gas discharge excimer or molecular fluorine laser chamber; a linenarrowing module within a first oscillator cavity; a laser amplificationstage containing an amplifying gain medium in a second gas dischargeexcimer or molecular fluorine laser chamber receiving the output of theseed laser oscillator and amplifying the output of the seed laseroscillator to form a laser system output comprising a laser output lightbeam of pulses; e.g., a MOPA or MOPO configured dual chamber seedlaser/amplifying laser system, such as applicants' assignee's MOPAXLA-XXX model laser systems, and further comprising a coherence bustingmechanism, of the kind(s) discussed herein, intermediate the seed laseroscillator and the amplifying gain medium stage. The amplification stagemay comprise a laser oscillation cavity. The amplification stagecomprising an optical path defining a fixed number of passes through theamplifying gain medium.

The laser system, e.g., for lithography use may operate within a matrixof MO operating conditions. The pulse energy of the output of the seedlaser oscillator being less than or equal to 0.1 mJ, or 0.2 mJ, or 0.5mJ, or 0.75 mJ. The ring power amplification stage may amplify theoutput of the broad band seed laser oscillator cavity to a pulse energyof over 1 mJ, or 2 mJ, or 5 mJ, or 10 mJ or 15 mJ. The laser system mayoperating at an output pulse repetition rate of up to 12 kHz, or ≧2 and≦8 kHz or ≧4 and ≦6 kHz. The system may comprise the seed laseroscillator containing a lasing gas comprising a mixture of fluorine andother gases and operating at ≦350 kPa of total lasing gas pressure, or≦300 kPa of total lasing gas pressure, or ≦250 kPa of total lasing gaspressure, or ≦200 kPa of total lasing gas pressure. The system maycomprise ≦35 kPa of fluorine partial pressure, or ≦30 kPa of fluorinepartial pressure, ≦25 kPa of fluorine partial pressure, or ≦20 kPa offluorine partial pressure.

Turning now to FIG. 32 there is shown in schematic form a pulsestretcher 160 a, which can be, e.g., a version of the optical pulsestretcher (“OPuS”) sold with applicants' assignee's laser systemshowever with, e.g., much shortened delay paths not designed for pulsestretching per se, i.e., enough stretching for significant pulseelongation in the spatial and temporal domains, e.g., increasing theT_(is) by 4× or more as in applicants' assignee's currently sold OPuSpulse stretchers. However, the same folding/inverse imaging effects onthe beam for coherency busting purposes, or also as explained in regardto the beam mixer of FIG. 31, can be achieved.

The coherency buster 160 a may have an input beam 162 a incident on abeam splitter 164 a, e.g., a partially reflective mirror 164 a for thepertinent wavelength. Part of the beam 162 a that is reflected into thedelay path comprised of a plurality of mirrors, e.g., confocal mirrors166 a, is negatively imaged, e.g., twice, and on the final leg of thedelay path pass through a pulse trimmer 170 a. It will be understoodthat such optical coherence busters may have more than four mirrors,e.g., six mirrors, but are illustrated schematically with only four forconvenience and clarity. A portion of the light exiting the pulsetrimmer 170 a is reflected into the output beam 172 a and a portionreenters the delay path. The delay path may be much shorter than theseven to ten meters or so of, e.g., a 4× OPus, such that the second andthird passes through the delay path do not substantially overlap thepulses entering and leaving the coherency buster 160 a, but rather donot even substantially stretch the pulses. There may be, however,overlapping in the high frequency components of the pulses, which servesin coherence busting. The pulse trimmer 160 a may be used, e.g., toshorten the ultimate output pulse 172 a, e.g., by cutting off a portionof the pulse circulating in the coherency buster delay path using thepulse trimmer 170 a, or much or all or substantially all of the secondand subsequent passes through the delay path. The pulse trimmer 170 amay be, e.g., a Pockels cell or other suitable fast acting lighttransmission switch, e.g., a light beam modulator/deflector, e.g., anelectro-optic or acousto-optic device, e.g., a crystal that changesrefractive index when excited by a field, e.g., an electric field, anacoustic field or a magnetic field.

FIG. 33 shows partly schematically and partly in block diagram form anexample of a coherence busting scheme 360 a and the results of aspectsof the scheme according to aspects of an embodiment of the disclosedsubject matter, e.g., in terms of beam divergence and thus coherencebusting. The illustrated system may incorporate, e.g., anoscillator/amplifier laser 370 a, e.g., including a solid state orexcimer seed laser 372 a, and an oscillator amplifier laser 394 a, orother power amplification stage, e.g., a ring power amplification stage.The amplifier gain medium 394 a may be, e.g., an excimer laser arrangedin a power oscillator configuration, e.g., with a fully reflective rearcavity mirror 396 a and an input/output coupler, e.g., 398 a. It will beunderstood that other seed laser/amplification stage arrangements, someof which are discussed herein, may also be used with the schematicallyillustrated coherence busting scheme shown by way of example in FIG. 33.

At the output of the seed laser 372 a is illustrated a representation ofthe seed laser output laser light pulse beam pulse coherency 374 acontaining a single dot indicative of relatively high coherency. Theoutput of the seed laser 372 a may be passed through one or morecoherency busters, e.g., 376 a, 378 a, e.g., as shown by example in FIG.32, or 1050 illustrated in FIG. 31 (discussed in more detail in theco-pending application noted above, 11/471,383 or other optical elementssuch as disclosed in US20050286599, referenced above, or one or moremini-OPuS coherence busting mechanisms discussed above, or combinationsthereof. A possible embodiment according to aspects of an embodiment ofthe disclosed subject matter may be the use of a confocal OPuS, e.g.,one like that disclosed in the co-pending U.S. patent application Ser.No. 10/847,799, entitled LASER OUTPUT LIGHT PULSE STRETCHER, filed onMay 18, 2004, referenced above, with, e.g., two confocal sphericalmirrors and four passes of delay path, i.e., from the beam splitter tomirror No. 1 to mirror No. 2 back to mirror No. 1 and back to mirror No.2 and then returned to the beam splitter, passing through, e.g., anoffset correction optic, e.g., as discussed in the co-pending U.S.patent application Ser. No. 11/394,512, entitled CONFOCAL PULSESTRETCHER, filed on Mar. 31, 2006, referenced above. This version of aso-called “mini-OpuS” may comprise two pulse stretchers in series, e.g.,with a delay path offset selected to slightly shift the high frequencypeaks in the temporal pulse intensity curve of the output of the masteroscillator, such that individual mini-peaks superimposed on the generalhumped or multi-humped shape of the output pulse from the MO becomeinterleaved in the treated pulse, which is beneficial in reducingspeckle. This may be achieved by, e.g., a delay offset of about 2 ns fora first 1 ns and then three ns delay line mini-OPuS pair or about a 1 nsdelay between a 3 ns and 4 ns delay line mini-OPuS pair in series or fora 4 ns and 5 ns delay line mini-OPus in series. It will be understoodthat the pulse itself will not be stretched significantly, e.g., to comeeven close to overlap other pulses, but rather will essentially not bestretched at all, since the delay path is so much shorter than the tenor so meters of delay path in the normal pulse stretching OPuSscurrently sold by applicants' assignee.

The preferred embodiment uses a first delay something more than Ins dueto increased alignment problems with the shorter delay and increasedaberrations in the pulse as stretched in a shorter delay path. Each ofthe delay paths is, however longer than the coherence length of thepulse and the second delay path is longer than the first, to achievecoherence busting, e.g., due to high frequency mini-peak interleavingeffects discussed herein.

The mini-OPuS pulse stretchers may be selected and arranged to, e.g.,fold the beam on itself or fan it out in first one axis, e.g., in afirst mini-OPus 376 a, resulting in the coherency representation 378 aand then in another orthogonally related axis, e.g., in a secondmini-OPuS 380 a, resulting, e.g., in the coherency representation 390 a.A pulse trimmer/pulse steerer 392 a, e.g., and electro-optical (“E-O”)element 392 a may sweep (paint) the seed beam into the input/outputcoupler 400 a of the amplifier portion 394 a resulting in the blurringin one axis as shown in the pulse coherency representation out of thepower oscillator 410 a (and also the coherence representation 410 intothe amplification gain stage 394 a). The “regular” or “standard” OPuS,e.g., a 4× T_(is) OPuS (roughly ten meters of delay path), which maycontain, e.g., 2 delay paths 412 a, 420 a initiated by a first beamsplitter 414 a and a second beam splitter 422 a, similarly may bearranged to fold the beam on itself in first one axis and then a secondresulting, e.g., in the pulse coherency representations of,respectively, 414 a and 424 a. The final coherency representation 424 ashows schematically that the coherency of the seed beam has been greatlyreduced, i.e., the beam has been smeared in its passage from the seedlaser 372 a to the amplifier gain medium 394 a and as amplified in theamplifier gain medium 394 a and subsequently further having itscoherency busted in the 4× regular OPuS 412 a, 420 a.

It will be understood by those skilled in the art that depending on theinitial coherency of the pulse, e.g., out of the seed laser, e.g.,almost completely coherent in the case of solid state seed lasers tovery little coherency, but still coherency that is desired to be evenfurther reduced, e.g., with an excimer seed laser the type, number andarrangement of coherency busting elements may vary. For example, it mayonly be necessary to do active coherency busting, e.g., with one form oranother of pulse steering/painting, for solid state seed lasers, andthis may in some cases for some applications prove to need only a rampor only AC pulse deflection, i.e., in one axis or the other, or mayprove to need both DC and AC pulse painting (Hybrid painting) along withOPuS effect coherency busting both between the MO and amplifier gainmedium, e.g., PO or PA or other amplification gain medium stage, e.g., aring power amplification stage, and also may need to employ the effectof the regular OPuS pulse stretcher(s) on the output of the amplifiergain medium. With an excimer gas discharge laser MO, with relativelymuch lower coherency than from a solid state seed laser, only passivecoherency busting, e.g., between the MO and gain amplifier medium may beneeded, e.g., with one or both of the mini-OPuSs 376, 380 or otherpassive optical elements as noted above between the MO and amplifiergain medium.

One may still need, however, to do beam steering also, e.g., with anactive beam steering mechanism for even more smearing of the pulse (moredivergence), that may be less essential and need a smaller sweepingangle. Such a seed laser mini-OPuS is believed to need approximatelyonly a 1 foot total path delay each and can also be conveniently builtonto the seed laser optical table as is currently the practice for relayoptics in applicants' assignee's XLA series laser systems.

FIG. 34 illustrates an exemplary relative speckle intensity for a 1 kVE-O deflector voltage v. relative timing. The relative standarddeviation curve 550 a is for 1 kV and the equivalent pulse curve iscurve 550 a′. A 2 kV E-O deflector voltage curve 552 a and equivalentpulse curve 552 a′ are also shown as is a 3 kV E-O deflector voltagecurve 554 a and equivalent pulse curve 554 a′. An example of a pointshift vs. E-O voltage curve 560 a is shown by way of example in FIG. 35.

According to aspects of an embodiment of the disclosed subject matter itis contemplated to apply a time changing voltage on a timescale similarto the seed pulse duration, e.g., by applying a DC voltage level untiltriggered, at which point the high voltage may be shorted to ground,e.g., via a stack of fast MOSFETS, e.g., illustrated schematically inFIG. 44 as a single transistor 1130 a. A plot of the applied voltage andthe seed laser pulse shape are shown in FIG. 36. Placing a seriesresister between the E-O cell terminal and voltage supply can be used tocontrol, e.g., the voltage slope applied to the E-O cell. The 50 pFcapacitance of the E-O cell in series with, e.g., a 200Ω resister givesan initial slope of about 10¹¹ μrad/s. The voltage across the E-O celldrops, e.g., as seen in FIG. 36 from the DC level to nearly zero in atime similar to the seed pulse duration. By changing the relative timingbetween the E-O cell pulser and the seed laser one can, e.g., change theamount of pointing sweep that occurs during the seed pulse. In addition,one can change the value of the initial DC voltage to effect a greateror lesser pointing sweep during the seed pulse. Applicants have testedthis fast pointing capability, e.g., with the seed laser only andreflecting from an OC only, therefore, with no OPuS effect from themultiple reflections from the OC and Rmax and no effects due to MOPOoperation. Without optimizing for relative timing between the E-O celland the seed pulse, applicants captured speckle patterns for a range oftiming between the two. Applicants applied three difference levels of DCvoltage to the E-O cell in order to change the maximum availablepointing slope. The results showed a minimum speckle intensitynormalized standard deviation at about 57 ns relative timing as seen,e.g., in FIG. 34. Without any angular shift during the seed pulse, atboth small and large relative timing values, below and above 57 ns thespeckle contrast is high. This correlates with values found byapplicants during static testing. When, e.g., the relative timing placesthe E-O Cell voltage slope coincident with the seed pulse, the specklepattern of a single pulse is smeared in the vertical direction, in adramatic and satisfactory way.

One can normalize these contrast values to the maximum value in order toevaluate the percentage reduction in contrast, e.g., brought about bythe dynamic pointing shift. At the optimum relative timing point thespeckle contrast was found to be reduced to about 40% of its peak. Usingthe 1/√{square root over (N)} assumption for equivalent number ofindependent pulses the data can be used to derive the number of pulsesrequired to achieve this level of speckle contrast reduction. At theoptimum relative timing, and with 3 kV applied to the E-O cell, thecontrast reduction was found to be equivalent to 6 pulses. Even highervoltage levels (and thus even larger pointing shift during a singlepulse) could improve this result. Applicants performed similarmeasurements with the seed laser pulse entering the MOPO amplificationstage cavity, but no discharges between the AMPLIFICATION STAGEelectrodes and noted that reflections from the OC and the Rmax in theXeF cavity, from the OPuS effect, beam spreading alone, indicated thatthe maximum speckle contrast was reduced by the amount predicted by theOPuS effect (N=1.56 with a 20% OC, giving 1/√{square root over(n)}=0.80. Thus 70% contrast becomes 56%). The effect of smearing, eventhough the initial speckle contrast is lower, appears not to change whenadding the secondary reflections from the full XeF cavity. Theequivalent pulse for speckle reduction is still about 6.

Applicants performed similar measurements with AMPLIFICATION STAGEcavity electrodes discharging and thus implicating the effects of theamplification within the AMPLIFICATION STAGE cavity, which indicated asshown in FIG. 34 the decrease in the impact on speckle reduction throughseed beam sweeping. With such a configuration, the effect was found tobe just over half of the equivalent number of pulses produced, i.e.,about 3, when operating as a MOPO, also found was a rather largereduction in peak speckle contrast, with no smearing. Previousmeasurements of MOPO operation showed a reduction equivalent to about 6pulses. These results show a reduction equivalent to about 8 pulses.Applicants suspect that the AMPLIFICATION STAGE cavity may discriminateagainst off-axis ray angles, e.g., in a flat-flat cavity, and thus thespray of angles sent into the cavity may not all be equally amplified(this could be corrected, e.g., with a true stable cavity, e.g.,employing a curved OC and a curved Rmax). Another explanation may bethat not all of the seed pulse takes part in controlling theAMPLIFICATION STAGE characteristics. Maybe only, e.g., the first 5 ns ofthe seed pulse's 10-15 ns pulse duration controls the AMPLIFICATIONSTAGE and thus the E-O sweep is not fast enough to occur within thatsmaller window. This may also be corrected, e.g., by using a smallerresister and a shorter sweep.

According to aspects of an embodiment of the disclosed subject matterapplicants propose to use a 6 mirror coherency busting mechanism (forconvenience herein optical pulse delay paths are indicated schematicallyas having four mirrors per delay path) which has been developed byapplicants' assignee for additional path delay inside either or both ofthe 1^(st) or 2^(nd) pulse stretchers in the OPuS used with applicants'assignee's XLA model multi-chamber laser systems. Such a delay path can,e.g., produce −1 imaging for each sub-pulse. This is illustratedschematically and in cartoon fashion, e.g., in FIG. 37A wherein isillustrated the summation of these “flipped” sub-pulses. The flippedsub-pulses shown, e.g., in FIG. 37B can be used, e.g., for improvedprofile uniformity and symmetry. A 6 mirror design can convert pointingshifts into a divergence increase which may, e.g., be beneficial in aring arrangement for ASE reduction. The standard 4 mirror design doesnot. It will be understood that the delay path for this coherencybusting purpose need not be as long as the actual OPuS used for pulsestretching to get a much increased pulse T_(is) e.g., forphotolithography uses. Rather the coherency busting mechanism, aso-called “mini-OPuS”, just needs to fold the pulses a certain number oftimes. This is illustrated by the pulse 580 a, with the corner(pre-flip) designated 582 a and the pulses 584 a, 586 a, 588 a. Inaddition, due to the almost inevitable misalignment of mirrors in thedelay path, a “hall of mirrors” or so-called OPuS effect, may alsoreduce the coherency in the seed laser pulse, and, e.g., so long as thedelay path exceeds the spatial coherency length of the beam furthercoherency busing occurs in the delay path. In this regard, a four mirrormini-OPuS, e.g., with confocal spherical mirrors for ease of alignment,may serve as a satisfactory coherency buster, even without beam flippingin both axis.

According to aspects of an embodiment of the disclosed subject matter itmay be necessary to combine two separate laser beams at various pointswithin a system according to aspects of an embodiment of the disclosedsubject matter. If only half of the entrance to a 6 mirror pulsestretcher is illuminated, the sub-pulses flip between top and bottom asshown, e.g., in FIG. 37B. The summation of these “flipped” sub-pulsescan lead to a filled in, full size profile, e.g., as illustrated in thepulse flipping simulation shown in FIG. 41, with the curve 562 a showingthe pulse before entering the delay path and curve 564 a (black) afterone delay path and 566 a (red) after a second delay path. Laserdivergence may then be used to fill in the center portion 568 a, e.g.,after some propagation, e.g., over about 1 m or so.

Use of a solid state laser source for lithography has been proposed inthe past and not pursued for two reasons. Solid state lasers are notconsidered capable of the high average power required for lithographyand a solid state laser produces single mode output which is highly(perfectly) coherent. According to aspects of an embodiment of thedisclosed subject matter applicants propose to address the low averagepower problem with, e.g., a hybrid solid state seed/excimer amplifiercombination. The high coherence properties of the solid state seed canbe addressed in a number of ways according to aspects of embodiments ofthe disclosed subject matter, e.g., by creating sub-pulses, e.g., thatare separated in time longer than the coherence length, or by, e.g.,changing the seed laser pointing, e.g., over very short time scales,e.g., within a single laser pulse, or a combination of both. Coherencybusting has been found by applicants to be of benefit in dual chambergas discharge (e.g. excimer) seed/gas discharge (e.g., excimer)amplifier portion lasers as well.

De-phasing of a speckle pattern can be seen from a diffuser 670 a tooccur with a λ/2d where d is the illumination distance for a slotaperture and diameter for a circular aperture, e.g., as illustratedschematically and in cartoon fashion in FIG. 38. Incoherence of aspeckle pattern can also be seen to occur from each sub-pulse producedby a pulse stretcher, which can, e.g., be further exploited by, e.g.,intentionally misaligning each pulse stretcher, e.g., a mirror(s) in thepulse stretcher, by a very slight amount. In point of fact, applicants'employer has discovered by testing that it is very hard to preciselyalign the mirrors in, e.g., an 4× T_(is) OPuS type of pulse stretcher,and they are slightly out of alignment almost all the time, withouthaving to intentionally misalign them. This amount of “ordinary”misalignment has been found by applicants employer to be an amountsufficient to achieve a desired level of speckle reduction and isillustrated schematically in FIG. 40, as discussed elsewhere.

The effective number of equivalent independent laser pulses can be seento be equal to the T_(is) magnification of the each pulse stretcher.Each OPuS pulse stretcher of the kind noted above may have amultiplication of around ˜2.4×. With, e.g., three stages of pulsestretching, the number of independent sub-pulses will be (2.4)³=13.8.Since speckle contrast scales with the number of independent sub-pulses,N, as 1/√N, pulse stretchers can provide an output speckle contrast of1√13.8=26.9% with an input speckle contrast of 100%. Since this maystill be too high a speckle contrast, according to aspects of anembodiment of the disclosed subject matter a mechanism(s) may beprovided to reduce the speckle contrast into or out of the pulsestretcher(s). The same can be said for the so-called mini-OPuSsdiscussed elsewhere.

Pulse trimming has been demonstrated, e.g., with the utilization ofelectro-optics, e.g., at 193 nm. Rather than polarization rotation, usedin some other forms of pulse trimming, electro-optics can be used forbeam steering, e.g., steering a seed laser light pulse beam within asingle pulse in the beam. Utilization of such, e.g., at the output ofthe seed laser, can result in, e.g., according to aspects of anembodiment of the disclosed subject matter, the electro-opticmaterial(s) only needing to be subject to a low average power seed laserbeam. By, e.g., randomly and/or continuously changing the beam steering,e.g., within a single laser pulse, the angular acceptance of the poweramplification stage can be “painted” or filled in for each laser pulse.As a result, a main pulse can have a divergence set, e.g., by theMO/power amplification stage optical configuration and not, e.g., by theseed laser characteristics. A greatly reduced coherence for the lasersystem output laser light pulse can be the result.

According to aspects of an embodiment of the disclosed subject matter aninjection controlled amplifier laser system, e.g., with a plane cavityand flat rear mirror, may have suitable energy stability, e.g., for seedpulse inject energies in the range of 0.0085 to 0.99 mJ. This energy ofthe beam may be, e.g., incident on the rear mirror of, e.g., a poweramplification stage, which may form the input coupler from the seedlaser. This reflector may have, e.g., about a 90% reflection and about8% transmission. Therefore, the seeding energy entering theamplification stage cavity itself may be, e.g., about an order ofmagnitude smaller than what is incident onto the back reflector. With aring cavity, especially with a partially reflecting seed injectionmechanism according to aspects of an embodiment of the disclosed subjectmatter, discussed elsewhere herein, e.g., the input seed energy may bemuch less wasted, e.g., admitting around 80% of the seed laser light. AnRmax and OC can be in an F₂ containing environment, and thus morerobust, though, e.g., if polarization coupling is used, couplingefficiency may still be less than optimum for certain applications. Asuitable architecture, e.g., in a MOPA configuration may be a 2-channel(“tic-toc”) solid state seed laser, e.g., a 3^(rd) harmonic Nd:YLF MO orNd:YAG system (tuned, e.g., to 351 nm) along with a pair of two 3-passXeF PA modules. Such a system in a MOPO, e.g., a master oscillator/poweramplification stage (such as a ring power oscillator amplificationstage) configuration is also considered as an effective alternative.Such a two channel MOPO approach may be similar to the MOPAconfiguration, i.e., with two seeded power oscillators. Various couplingtechniques could be used, e.g., MO coupling using a polarizationtechnique or a seed inject mechanism. Efficiency v. E_(mo) for differingPO/PA configurations has been found to be better for a MOPO or a threepass MOPA, though four pass MOPAs were not tested. Exemplary pulse width(FWHM) has been found to be for an MOPO about 17.3 ns, for a MOPA,single pass, about 13.9 ns and for a MOPA triple pass about 12.7 ns.

Applicants have examined speckle patters for decorrelation with angularshift, e.g., in a MOPO output beam, e.g., with a Nd-YLF seed laser and aXeF power oscillator (e.g., a flat-flat polarization coupledarrangement). With the relative timing between the XeF discharge and theseed laser pulse adjusted and angular and spatial adjustment also madefor maximum suppression of the weak line (353) produced by the XeF gain.

The maximum intensity of the seed pulse has been observed to occurduring the initial, very low level, fluorescence of the amplificationstage. This very low level fluorescence (and thus gain) is believed tobe enhanced by this seed light, as observed in MOPO output. Adjustmentof the timing of the seed earlier than or later than, e.g., about 20 orso ns before the amplification stage firing can, e.g., lead to anincrease in weak line output, an indication of, e.g., when the“primordial” photons are generated in the AMPLIFICATION STAGE.

FIG. 39 illustrates schematically the results of a coherency bustingscheme on an output laser pulse, e.g., in relation to a scanneracceptance window, e.g., introducing horizontal and vertical (asillustrated in the plane of the page drawing of FIG. 39) directions. Thedot 780 a illustrated schematically and by way of example an initialseed laser output pulse profile 780 a. The pattern of pulses 782 aillustrate a pattern of sub-pulse profiles 782 a after beam folding in aperfectly aligned beam delay path, or through a misaligned beam delaypath or both, or a combination thereof, and the circles 784 a aroundeach represent the effect on the profile of electro-optical smearing.

Turning now to FIG. 40 there is shown a schematic representation of theeffects of coherence busting according to aspects of an embodiment ofthe disclosed subject matter. Utilizing an imaging delay path, e.g., apulse stretcher, e.g., a so-called optical pulse stretcher (“OPuS”),e.g., a 4× T_(is) six mirror OPuS sold with the above noted applicants'assignee's laser systems, and illustrated in United States patents andco-pending applications noted above, or a modified version thereof witha shorter delay path used, e.g., for folding the beam on itself and/orfor delay exceeding the coherence length as discussed above, theso-called mini-OPuS, one can achieve a degree of coherence busting,e.g., between the MO and amplifier gain medium, e.g., a PA or a PO or aring power amplification stage. Other forms of coherence busting e.g.,as illustrated in FIG. 31 could be used alone or in combination withsuch a “mini-OPuS,” e.g., as illustrated in FIG. 33 and elsewhereherein.

According to aspects of an embodiment of the disclosed subject matter,the pointing/divergence sensitivity of a pulse stretcher, e.g., a 4mirror 6 mirror pulse stretcher, e.g., a regular OPuS such as a 4×T_(is) OPuS, or a so-called mini-OPuS, or the delay path discussed inmore detail in regard to FIG. 31, can be put to advantage, e.g., byadding active mirror control with feedback from, e.g., apointing/divergence sensor, illustrated, e.g., in FIGS. 13, 14 and 42.Such advantages include creating, e.g., a hall of mirrors effectwhereby, e.g., the laser output light pulse beam being smoothed in thedelay path and, further, actually becomes something like a plurality ofbeams of very slightly different pointing and thus angles of incidenceon the various mirrors of the pulse stretcher. Applicants assignee hasobserved this in pulse stretchers where it is very difficult toperfectly align the mirrors, e.g., of the currently used 4× T_(is) OPuSpulse stretcher, thus creating the hall of mirrors effect that reducesthe coherence of the laser output light pulse beam exiting the pulsestretcher. Thus the beam 860 a forms a plurality of separate beams 82 a.In FIG. 40 this is also illustrated schematically and as a result of aflat-flat cavity 850 a with slightly misaligned mirrors forming the rearof the cavity 852 a and an output coupler 854 a, but the same effect hasbeen observed in an OPuS by applicants employer with the coherencebusting effect noted above. The cavity illustrated in FIG. 40 may alsohave a polarizing input coupler 858 a and a quarter wave plate 856 a.

FIG. 40 illustrates a reduction in coherency, e.g., when using both thereflectivity of an OC and an Rmax, e.g., in a flat-flat cavity with,e.g., a polarizing input coupling from a seed laser source of seed laserpulses. The angles have been exaggerated for clarity of illustration.There are, e.g., multiple rays produced by a static fan out, i.e., “hallof mirrors” effect, e.g., created between the OC and the Rmax. Thetheoretical energy weighting of these rays, assuming no transmissionlosses through the cavity and perfect reflectivity is shown below.

Ray Number Fractional Energy Normalized Energy 1 0.2 = 0.200 0.3125 20.8*0.8 = 0.640 1.000 3 0.8*0.2*0.8 = 0.128 0.2000 4 0.8*0.2*0.2*0.8 =0.0256 0.0400 5 0.8*0.2*0.2*0.2*0.8 = 0.00512 0.0080 60.8*0.2*0.2*0.2*0.2*0.8 = 0.00102 0.0016One may assume that each ray is incoherent from all others, e.g., wherethe path length between the OC and the Rmax is maintained to be longerthan the temporal coherence length. Each ray may also be assumed, e.g.,to be angled slightly different from all others since, e.g., perfectalignment is believed to be extremely difficult, especially in thevertical direction. Applicants believe that about 37 μrad of angledifference in the vertical direction is needed to create uncorrelatedspeckle. Summing the normalized energy weighting to give the equivalentnumber of independent pulses and taking the square root to give thereduction in standard deviation, the sum from the above is 1.56. Thesquare root is 1.25 and thus the standard deviation when using both OCand Rmax reflections is predicted to be 0.551/1.25=0.440, which comportswell with a value that applicants have measured, i.e., 0.427.

Static fan out, otherwise referred to herein as a hall of mirrorseffect, believed to be essentially unavoidable with manual alignment,produces a single pulse speckle contrast with amplification in anamplification gain medium that is 2.50× smaller than the seed laseralone. This reduction is the equivalent of 6.3 uncorrelated sub-pulses.Some of this contrast reduction is due to the weak line content from theXeF power oscillator used for testing the effects of the oscillationamplification stage, but most is believed to be due to the static fanout effect. Likely, many of the sub-pulses created by the OPuS-likestatic fan out characteristics of the OC-Rmax (OC-rear cavity mirror)reflections are all amplified to nearly equal intensities and thuscreate more equivalent independent pulses than shown in the above table.

Tilt angle required to produce uncorrelated speckle patterns may besignificant. The first big jump in equivalent pulses, from 1.0 to 1.55,is believed by applicants to be mostly due to the poor pulse-to-pulserepeatability of the speckle patterns when running as a MOPO. Evenwithout changing the mirror tilt at all, two pulses are correlated nobetter than 30-35%. With seed only, this pulse-to-pulse correlation hasbeen found to be about 85-90%. The long slow rise in equivalent pulsenumber does not even reach a value of 2.0 until about 400 μrad of mirrortilt as illustrated, e.g., in FIG. 46. This result could mean, e.g.,there may be a need for a large angular sweep, of about ±500-1000 μrad,e.g., to create several uncorrelated speckle patterns in a single pulse.

Through experimentation relating to coherence applicants' employer haslearned that, e.g., sub-pulses produced by a pulse stretcher areincoherent and lead to a different fringe pattern if their angles areslightly shifted. The pin hole fringe pattern shifts maximum to minimumwhen input angle is λ/2d.

A plot of pointing shift (inferred by applicants from speckle shiftmeasurements) v. E-O cell applied voltage is shown in FIG. 35. Accordingto aspects of an embodiment of the disclosed subject matter applicantspropose to sweep the pointing of the seed laser within a single pulse inorder to reduce the speckle contrast within. This may be done, e.g.,with electro optical elements, e.g., element 392 a illustratedschematically in FIG. 33. Using vertical expansion prior to input of aseed laser pulse into an excimer power oscillator, e.g., a XeF chamber,placed as close to an input coupler, e.g., a beam splitter, and with aclear aperture of the E-O deflector at around 3.2 mm in diameter, thedeflector may have to be upstream of the vertical expansion (not shownin FIG. 33). To minimize any translation in the oscillator cavity, e.g.,associated with the angular tilt from the E-O deflector, it may bedesirable to place the E-O deflector as close to the amplifier cavity aspossible.

Turning now to FIG. 42 there is shown schematically and partly in blockdiagram form a beam combiner system 600 a, according to aspects of anembodiment of the disclosed subject matter. The beam combiner system 600a may include, e.g., a first amplifier gain medium portion 602 a and asecond amplifier gain medium portion 604 a, each of which may be, e.g.,a PA or PO or ring power amplification stage, as described elsewhere inthe present application. The output of each of the amplifier portions602 a, 604 a may pass through a beam expander 608, which may include aprism 610 a and a prism 612 a, e.g., magnifying the beam by 2×. Aturning mirror 620 a may steer a first laser system output light pulsebeam 622 a from the amplifier 602 a to a second turning mirror 624 awhich may steer the pulse beam 622 a to form a pulse beam 632 a onto abeam splitter for a first pulse stretcher 640 a and thence to a beamsplitter 646 a for a second pulse stretcher 644 a. A turning mirror 630a may steer a second laser system output light pulse beam 632 a from thesecond amplifier 604 a to a second turning mirror 634 a, which may steerthe beam 632 a to form a beam 634 a to be incident on the beam splitter642 a and thence the beam splitter 646 a. The output of the first OPuSand second OPuS, which may be “mini-OPuSs” as discussed elsewhere in thepresent application, may pass through another beam splitter 650 a,where, e.g., a small portion of the laser system output laser lightpulse beam may be diverted, e.g., for metrology purposes, e.g., focusedby a focusing lens 652 a into a divergence detector 654 a, which may bepart of a control system (not shown) providing feedback control signals656 a, e.g., to the beam splitters 642 a, 646 a of the first and/orsecond OPuSs 640 a, 644 a or the turning mirrors for each of the beams632 a, 634 a to increase or decrease divergence. Such coherency bustingmay be at the input to the amplifiers 602 a, 604 a, e.g., shown in FIG.42 as opposed to the outputs.

FIG. 43 gives an example of an idealized high frequency painting E-Ovoltage signal superimposed on a ramped (time varying) E-O DC voltagesignal in relation to the intensity of the seed pulse being “painted”,e.g., into a delay path or into the amplifying gain medium, e.g., a PAor PO or other power amplification stage. The ramp voltage may becreated, e.g., by a fast R-C decay of an E-O cell capacitance asillustrated schematically in the circuit of FIG. 45. Due to certainconstraints on a test circuit that applicants have so far built andtested, e.g., limited RF frequency, impedance mismatch, E-O load cellcapacitance mismatch and the like, the actual voltages delivered by the“painting” circuit are shown in FIG. 44, as best as could be measuredconsidering difficulties with probe loading, etc. These areapproximately 25% of the needed RF frequency (e.g., about 100 MHz asopposed to 400 MHz) and 10% of the needed peak to peak voltage (e.g.,around ±200 kV as opposed to ±2000 kV). The painting voltages could, ofcourse, be better optimized, however, the test circuit was used todemonstrate the effectiveness of “painting” the seed beam into theamplifier gain medium for coherency/speckle reduction, e.g., with hybridpainting using both time varying DC steering and AC modulation, e.g.,one in one axis and the other in a second axis, e.g., orthogonallyrelated to each other.

Applicants experimental measurements have determined that with no rampand no AC voltage the 2D speckle contrast overall is 76.8% and variesfrom the horizontal to the vertical axis. With painting using the rampalone the speckle contrast overall was 29.4%, again varying in the twoaxes. Painting with the AC alone gave a speckle contrast overall of59.9%, again varying in the two axes. With the ramp and AC voltagesapplied the spectral contrast was 28.1% overall and varying in bothaxes. This was using a less optimized circuit than the one of FIG. 40,which was not available for the testing and the actual tested circuittest results are shown in FIG. 44.

Applicants believe that a more optimized circuit, shown by way ofexample in FIG. 45, will even improve further the reduction in specklecontrast. The circuit 1100 a of FIG. 40 may include, e.g., an E-O cell,such as noted above, with an E-O cell capacitance 1104 a and animpedance matching inductor 1110 a, and an N:1 step-up transformer 1120a. Also included as illustrated may be, e.g., a DC power supply 1122 acharging a capacitor 1126 a through a large resistor 1130 a and an RFfrequency generator connected to a fast acting switch, e.g., atransistor 1140 a (in reality a bank of such transistors in parallel),through a resistor. Also the capacitor 1126 a discharges through a smallresistor 1142 a when the switch 1140 a is closed.

Turning to FIG. 47 there is illustrated schematically and in blockdiagram form a laser treatment system, e.g., and LTPS or tbSLS laserannealing system, e.g., for melting and recrystallizing amorphoussilicon on sheets of glass substrates at low temperature. The system1070 may include, e.g., a laser system 20 such as described herein and aoptical system 1272 to transform the laser 20 output light pulse beamfrom about 5×12 mm to 10 or so microns×390 mm or longer thin beams fortreating a workpiece, e.g., held on a work piece handling stage 1274.

It will be understood by those skilled in the art that disclosed hereinis a method and apparatus which may comprise a line narrowed pulsedexcimer or molecular fluorine gas discharge laser system which maycomprise a seed laser oscillator producing an output comprising a laseroutput light beam of pulses which may comprise a first gas dischargeexcimer or molecular fluorine laser chamber; a line narrowing modulewithin a first oscillator cavity; a laser amplification stage containingan amplifying gain medium in a second gas discharge excimer or molecularfluorine laser chamber receiving the output of the seed laser oscillatorand amplifying the output of the seed laser oscillator to form a lasersystem output comprising a laser output light beam of pulses, which maycomprise a ring power amplification stage. The ring power amplificationstage may comprise an injection mechanism which may comprise a partiallyreflecting optical element, e.g., a beam splitter through which the seedlaser oscillator output light beam is injected into the ring poweramplification stage. The ring power amplification stage may comprise abow-tie loop or a race track loop, such that, e.g., the seed beam passesthrough a portion of the amplification stage gain medium in opposingdirections in each loop of the laser oscillation in the amplificationstage. The pulse energy of the output of the seed laser oscillator maybe less than or equal to 0.1 mJ, or 0.2 mJ, or 0.5 mJ, or 0.75 mJ. Thering power amplification stage may amplify the output of the seed laseroscillator cavity to a pulse energy of ≧1 mJ, or ≧2 mJ, or ≧5 mJ, or 10mJ, or ≧15 mJ. The laser system may operate at an output pulserepetition rate of up to 12 kHz, or ≧2 and ≦8 kHz, or ≧4 and ≦6 kHz. Theapparatus and method may comprise a broad band pulsed excimer ormolecular fluorine gas discharge laser system which may comprise a seedlaser oscillator producing an output comprising a laser output lightbeam of pulses which may comprise a first gas discharge excimer ormolecular fluorine laser chamber; a laser amplification stage containingan amplifying gain medium in a second gas discharge excimer or molecularfluorine laser chamber receiving the output of the seed laser oscillatorand amplifying the output of the seed laser oscillator to form a lasersystem output which may comprise a laser output light beam of pulses,which may comprise a ring power amplification stage. The ring poweramplification stage may comprise an injection mechanism comprising apartially reflecting optical element through which the seed laseroscillator output light beam is injected into the ring poweramplification stage. The ring power amplification stage may comprise abow-tie loop or a race track loop. The apparatus and method may comprisea coherence busting mechanism intermediate the seed laser oscillator andthe amplifier gain medium. The coherence busting mechanism may comprisean optical delay path having a delay length longer than the coherencelength of a pulse in the seed laser oscillator laser output light beamof pulses. The optical delay path may not substantially increase thelength of the pulse in the seed laser oscillator laser output light beamof pulses, however, the delay is long enough to, e.g., interleave higherfrequency elements of the seed pulse but not create overlapping pulses,e.g., as occurs in a 4× OPuS sold by applicants' assignee, with a delaypath of many meters, which also significantly increases the T_(is) ofthe pulse as well as its temporal and spatial length. The coherencebusing mechanism may comprise a first optical delay path of a firstlength and a second optical delay path of a second length, with theoptical delay in each of the first and second delay paths exceeding thecoherence length of a pulse in the seed laser oscillator laser outputlight beam of pulses, but not substantially increasing the length of thepulse, and the difference in the length of the first delay path and thesecond delay path exceeding the coherence length of the pulse and alsonot substantially increasing the length of the pulse. The apparatus andmethod may comprise a line narrowed pulsed excimer or molecular fluorinegas discharge laser system that may comprise a seed laser oscillatorproducing an output comprising a laser output light beam of pulses thatmay comprise a first gas discharge excimer or molecular fluorine laserchamber; a line narrowing module within a first oscillator cavity; alaser amplification stage containing an amplifying gain medium in asecond gas discharge excimer or molecular fluorine laser chamberreceiving the output of the seed laser oscillator and amplifying theoutput of the seed laser oscillator to form a laser system outputcomprising a laser output light beam of pulses, which may comprise aring power amplification stage; a coherence busting mechanismintermediate the seed laser oscillator and the ring power amplificationstage. The ring power amplification stage may comprise an injectionmechanism comprising a partially reflecting optical element throughwhich the seed laser oscillator output light beam is injected into thering power amplification stage. The coherence busting mechanism maycomprise an optical delay path having a delay length longer than thecoherence length of a pulse in the seed laser oscillator laser outputlight beam of pulses. The optical delay path may not substantiallyincrease the length of the pulse in the seed laser oscillator laseroutput light beam of pulses. The coherence busting mechanism maycomprise a first optical delay path of a first length and a secondoptical delay path of a second length, with the optical delay in each ofthe first and second delay paths exceeding the coherence length of apulse in the seed laser oscillator laser output light beam of pulses,but not substantially increasing the length of the pulse, and thedifference in the length of the first delay path and the second delaypath exceeding the coherence length of the pulse and also notsubstantially increasing the length of the pulse. The coherence bustingmechanism may comprise a coherence busting optical delay structuregenerating multiple sub-pulses delayed sequentially from a single inputpulse, wherein each sub-pulse is delayed from the following sub-pulse bymore than the coherence length of the pulse light. The apparatus andmethod may comprise a broad band pulsed excimer or molecular fluorinegas discharge laser system which may comprise a seed laser oscillatorproducing an output comprising a laser output light beam of pulses whichmay comprise a first gas discharge excimer or molecular fluorine laserchamber; a laser amplification stage containing an amplifying gainmedium in a second gas discharge excimer or molecular fluorine laserchamber receiving the output of the seed laser oscillator and amplifyingthe output of the seed laser oscillator to form a laser system outputcomprising a laser output light beam of pulses, which may comprise aring power amplification stage; a coherence busting mechanismintermediate the seed laser oscillator and the ring power amplificationstage. The ring power amplification stage may comprise an injectionmechanism comprising a partially reflecting optical element throughwhich the seed laser oscillator output light beam is injected into thering power amplification stage. The coherence busting mechanism maycomprise an optical delay path having a delay length longer than thecoherence length of a pulse in the seed laser oscillator laser outputlight beam of pulses. The optical delay path may not substantiallyincrease the length of the pulse in the seed laser oscillator laseroutput light beam of pulses. The coherence busing mechanism may comprisea first optical delay path of a first length and a second optical delaypath of a second length, with the optical delay in each of the first andsecond delay paths exceeding the coherence length of a pulse in the seedlaser oscillator laser output light beam of pulses, but notsubstantially increasing the length of the pulse, and the difference inthe length of the first delay path and the second delay path exceedingthe coherence length of the pulse and also not substantially increasingthe length of the pulse. The coherence busting mechanism comprising acoherence busting optical delay structure generating multiple sub-pulsesdelayed sequentially from a single input pulse, wherein each sub-pulseis delayed from the following sub-pulse by more than the coherencelength of the pulse light. The apparatus and method may comprise apulsed excimer or molecular fluorine gas discharge laser system whichmay comprise a seed laser oscillator producing an output comprising alaser output light beam of pulses which may comprise a first gasdischarge excimer or molecular fluorine laser chamber; a line narrowingmodule within a first oscillator cavity; a laser amplification stagecontaining an amplifying gain medium in a second gas discharge excimeror molecular fluorine laser chamber receiving the output of the seedlaser oscillator and amplifying the output of the seed laser oscillatorto form a laser system output comprising a laser output light beam ofpulses; a coherence busting mechanism intermediate the seed laseroscillator and the laser amplification stage comprising an optical delaypath exceeding the coherence length of the seed laser output light beampulses. The amplification stage may comprise a laser oscillation cavity.The amplification stage may comprise an optical path defining a fixednumber of passes through the amplifying gain medium. The coherencebusting mechanism may comprise an optical delay path having a delaylength longer than the coherence length of a pulse in the seed laseroscillator laser output light beam of pulses. The optical delay path maynot substantially increase the length of the pulse in the seed laseroscillator laser output light beam of pulses. The coherence bustingmechanism may comprise a first optical delay path of a first length anda second optical delay path of a second length, with the optical delayin each of the first and second delay paths exceeding the coherencelength of a pulse in the seed laser oscillator laser output light beamof pulses, but not substantially increasing the length of the pulse, andthe difference in the length of the first delay path and the seconddelay path exceeding the coherence length of the pulse and also notsubstantially increasing the length of the pulse.

Applicants have simulated through calculations speckle reduction asrelates to the location of coherence lengths within a single gasdischarge (e.g., ArF or KrF excimer) laser system output pulse aftersuch a pulse has passed through the two OPuS pulse stretchers sold onlaser systems manufactured by applicants' assignee Cymer, Inc., used forpulse stretching to increase the total integrated spectrum (T_(is)) toreduce the impact of peak intensity in the laser output pulse on theoptics in the tool using the output light from the laser system, e.g., alithography tool scanner illuminator. There are two OPuS in series, withthe first having a delay path sufficient to stretch the T_(is) of theoutput pulse from about 18.6 ns to about 47.8 ns and the second tostretch the pulse further to about 83.5 ns, e.g., measured at E955 (thewidth of the spectrum within which is contained 95% of the energy of thepulse.

Starting with the unstretched pulse, applicants divided the pulse intoportions equal to the approximate coherence length, assuming a FWHMbandwidth of 0.10 pm and a Gaussian shape for the coherence lengthfunction. The impact of the pulse stretching on the coherence lengthportions of the pulse after passing through the first OPuS was to showthat a first intensity hump in the spectrum of the stretched pulse wasmade up of the coherence length portions of the main pulse, a secondintensity hump was mad up of coherence length portions of the main pulseoverlapped with coherence length portions of a first daughter pulse. Athird hump in the intensity spectrum is the result of overlapping of thefirst and second daughter pulses. Looking at the individual coherencelength portions of the two humps applicants observed that the multipleversions (including daughters) of the coherence length portions remainedsufficiently separated to not interfere with each other.

After passage through the second OPuS the simulated spectra, again onlylooking at the content of the first three humps in the stretched pulse,in the simulation (under the second hump were contributions from theoriginal undelayed pulse, as before, the first delayed pulse from thefirst OPuS, as before and the first delayed pulse from the second OPuS),applicants observed that in this second pulse the multiple versions ofthe coherence length portions were very close together. This is causedby the fact that the first OPuS has a delay of ˜18 ns and the second hasa delay of ˜22 ns. Thus only ˜4 ns separates the versions of thecoherence length portions, which is still not close enough forinterference.

Under the third hump applicants observed contributions from the firstdelayed pulse from the first OPuS, the second delayed pulse from firstOPuS, the first delayed pulse from the second OPuS, and the seconddelayed pulse from second OPuS. Applicants observed that the separationbetween some related coherence portions is larger than for others in thethird hump in the intensity spectrum of the pulse stretched by twoOPuSs. This increase in separation is due to the fact that two roundtrips through each OPuS equal ˜36 ns=18*2 and ˜44 ns=22*2. Thus theseparation between coherence lengths grows with each round trip.

Applicants concluded that for a mini-OPuS as described in thisapplication a single mini-OPuS with delay equal to one coherence lengthwill create a train of pulses that dies out after about 4 coherencelength values. Thus, applicants determined that for a single mini-OPuSto be effective, the two main OPuSs should not bring any daughtercoherence lengths to within 4 coherence lengths of each other. But,applicants have observed in the simulation that the main OPuSs do justthat, though only marginally so. The separation between coherencelengths for the third and greater humps is sufficient. Applicantsbelieve that the impact of a single mini-OPuS between MO andamplification gain medium will be nearly the full expected coherencebusting effect. A second mini-OPuS between MO and PA may not adequatelyinteract with the two main OPuss. The empty spaces, not filled withrelated coherence length portions of the spectra pulse humps get morescarce when one combines a single min-OPuS and two regular OPuSs, andthe second may be too much. According to aspects of an embodiment of thepresent invention applicants propose the coordinated change of theregular OPuS delay lengths when the mini-OPuS(s) are installed,including whether they are part of the laser system or installed downstream of the regular main OPuSs, e.g., in the lithography tool itself.Applicants believe that such mini-OPuS(s) can fill in the valleys of thepulse duration somewhat, leading to an increase in T_(is), e.g.,allowing a reduction in the delay lengths of one of the two main OPuSesfor better overall coherence length separation.

While the particular aspects of embodiment(s) of the LASER SYSTEMdescribed and illustrated in this patent application in the detailrequired to satisfy 35 U.S.C. §112 is fully capable of attaining anyabove-described purposes for, problems to be solved by or any otherreasons for or objects of the aspects of an embodiment(s) abovedescribed, it is to be understood by those skilled in the art that it isthe presently described aspects of the described embodiment(s) of thesubject matter disclosed are merely exemplary, illustrative andrepresentative of the subject matter which is broadly contemplated bythe subject matter disclosed. The scope of the presently described andclaimed aspects of embodiments fully encompasses other embodiments whichmay now be or may become obvious to those skilled in the art based onthe teachings of the Specification. The scope of the present LASERSYSTEM is solely and completely limited by only the appended claims andnothing beyond the recitations of the appended claims. Reference to anelement in such claims in the singular is not intended to mean nor shallit mean in interpreting such claim element “one and only one” unlessexplicitly so stated, but rather “one or more”. All structural andfunctional equivalents to any of the elements of the above-describedaspects of an embodiment(s) that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the present claims. Anyterm used in the specification and/or in the claims and expressly givena meaning in the Specification and/or claims in the present applicationshall have that meaning, regardless of any dictionary or other commonlyused meaning for such a term. It is not intended or necessary for adevice or method discussed in the Specification as any aspect of anembodiment to address each and every problem sought to be solved by theaspects of embodiments disclosed in this application, for it to beencompassed by the present claims. No element, component, or method stepin the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element in the appendedclaims is to be construed under the provisions of 35 U.S.C. §112, sixthparagraph, unless the element is expressly recited using the phrase“means for” or, in the case of a method claim, the element is recited asa “step” instead of an “act”.

It will be understood also be those skilled in the art that, infulfillment of the patent statutes of the United States, applicant(s)has disclosed at least one enabling and working embodiment of eachinvention recited in any respective claim appended to the Specificationin the present application and perhaps in some cases only one. Forpurposes of cutting down on patent application length and drafting timeand making the present patent application more readable to theinventor(s) and others, applicant(s) has used from time to time orthroughout the present application definitive verbs (e.g., “is”, “are”,“does”, “has”, “includes” or the like) and/or other definitive verbs(e.g., “produces,” “causes” “samples,” “reads,” “signals” or the like)and/or gerunds (e.g., “producing,” “using,” “taking,” “keeping,”“making,” “determining,” “measuring,” “calculating” or the like), indefining an aspect/feature/element of, an action of or functionality of,and/or describing any other definition of an aspect/feature/element ofan embodiment of the subject matter being disclosed. Wherever any suchdefinitive word or phrase or the like is used to describe anaspect/feature/element of any of the one or more embodiments disclosedherein, i.e., any feature, element, system, sub-system, component,sub-component, process or algorithm step, particular material, or thelike, it should be read, for purposes of interpreting the scope of thesubject matter of what applicant(s) has invented, and claimed, to bepreceded by one or more, or all, of the following limiting phrases, “byway of example,” “for example,” “as an example,” “illustratively only,”“by way of illustration only,” etc., and/or to include any one or more,or all, of the phrases “may be,” “can be”, “might be,” “could be” andthe like. All such features, elements, steps, materials and the likeshould be considered to be described only as a possible aspect of theone or more disclosed embodiments and not as the sole possibleimplementation of any one or more aspects/features/elements of anyembodiments and/or the sole possible embodiment of the subject matter ofwhat is claimed, even if, in fulfillment of the requirements of thepatent statutes, applicant(s) has disclosed only a single enablingexample of any such aspect/feature/element of an embodiment or of anyembodiment of the subject matter of what is claimed. Unless expresslyand specifically so stated in the present application or the prosecutionof this application, that applicant(s) believes that a particularaspect/feature/element of any disclosed embodiment or any particulardisclosed embodiment of the subject matter of what is claimed, amountsto the one an only way to implement the subject matter of what isclaimed or any aspect/feature/element recited in any such claim,applicant(s) does not intend that any description of any disclosedaspect/feature/element of any disclosed embodiment of the subject matterof what is claimed in the present patent application or the entireembodiment shall be interpreted to be such one and only way to implementthe subject matter of what is claimed or any aspect/feature/elementthereof, and to thus limit any claim which is broad enough to cover anysuch disclosed implementation along with other possible implementationsof the subject matter of what is claimed, to such disclosedaspect/feature/element of such disclosed embodiment or such disclosedembodiment. Applicant(s) specifically, expressly and unequivocallyintends that any claim that has depending from it a dependent claim withany further detail of any aspect/feature/element, step, or the like ofthe subject matter of what is claimed recited in the parent claim orclaims from which it directly or indirectly depends, shall beinterpreted to mean that the recitation in the parent claim(s) was broadenough to cover the further detail in the dependent claim along withother implementations and that the further detail was not the only wayto implement the aspect/feature/element claimed in any such parentclaim(s), and thus be limited to the further detail of any suchaspect/feature/element recited in any such dependent claim to in any waylimit the scope of the broader aspect/feature/element of any such parentclaim, including by incorporating the further detail of the dependentclaim into the parent claim.

It will be understood by those skilled in the art that the aspects ofembodiments of the subject matter disclosed above are intended to bepreferred embodiments only and not to limit the disclosure of thesubject matter disclosed(s) in any way and particularly not to aspecific preferred embodiment alone. Many changes and modification canbe made to the disclosed aspects of embodiments of the disclosed subjectmatter disclosed(s) that will be understood and appreciated by thoseskilled in the art. The appended claims are intended in scope andmeaning to cover not only the disclosed aspects of embodiments of thesubject matter disclosed(s) but also such equivalents and othermodifications and changes that would be apparent to those skilled in theart. In additions to changes and modifications to the disclosed andclaimed aspects of embodiments of the subject matter disclosed(s) notedabove others could be implemented.

1. A laser apparatus comprising: a seed laser oscillator producing anoutput comprising a laser output light beam of pulses, the seed laseroscillator comprising: a first gas discharge excimer or molecularfluorine laser chamber; a laser amplification stage containing anamplifying gain medium in a second gas discharge excimer or molecularfluorine laser chamber receiving the output of the seed laser oscillatorand amplifying the output of the seed laser oscillator to form a lasersystem output comprising a laser output light beam of pulses, the laseramplification stage comprising: a ring power amplification stagecomprising: a beam expander; and a partially reflecting optical elementthrough which the seed laser oscillator output light beam is injectedinto the ring power amplification stage; and a coherence bustingmechanism intermediate the seed laser oscillator and the ring poweramplification stage comprising an optical delay path having a delaylength longer than the coherence length of a pulse in the seed laseroscillator laser output light beam of pulses; wherein the beam expanderis configured to de-magnify the laser beam as the laser beam travelsfrom the partially reflecting optical element toward the amplifying gainmedium and to expand the laser beam as the laser beam travels from theamplifying gain medium toward the partially reflecting optical element.2. The apparatus of claim 1 further comprising: the ring poweramplification stage comprising a bow-tie loop.
 3. The apparatus of claim1 further comprising: the ring power amplification stage comprising arace track loop.
 4. The apparatus of claim 1 further comprising: thepulse energy of the output of the seed laser oscillator being less thanor equal to 1.0 mJ.
 5. The apparatus of claim 1 further comprising: thepulse energy of the output of the seed laser oscillator being less thanor equal to 0.1 mJ.
 6. The apparatus of claim 1 further comprising: thering power amplification stage amplifying the output of the seed laseroscillator cavity to a pulse energy of ≧5 mJ.
 7. The apparatus of claim1 further comprising: the ring power amplification stage amplifying theoutput of the seed laser oscillator to a pulse energy of ≧15 mJ.
 8. Theapparatus of claim 1 further comprising: the laser system operating atan output pulse repetition rate of up to 12 kHz.
 9. The apparatus ofclaim 1 further comprising: the laser system operating at an outputpulse repetition rate of ≧4 and ≦6 kHz.
 10. The apparatus of claim 1wherein the beam expander comprises at least one prism.
 11. Theapparatus of claim 1 wherein the beam expander comprises a two prismbeam expander.
 12. The apparatus of claim 1 wherein the ring poweramplification stage is defined between a beam reverser and the partiallyreflecting optical element.
 13. The apparatus of claim 12 wherein thebeam reverser comprises a prism that is configured to cross a light beamtraveling along a first path through the amplifying gain medium with alight beam traveling along a second path through the amplifying gainmedium.
 14. The apparatus of claim 1 wherein the seed laser oscillatorcomprises a line narrowing module within a first oscillator cavity. 15.A laser apparatus comprising: a seed laser oscillator producing anoutput comprising a laser output light beam of pulses, the seed laseroscillator comprising: a first gas discharge excimer or molecularfluorine laser chamber; a laser amplification stage containing anamplifying gain medium in a second gas discharge excimer or molecularfluorine laser chamber receiving the output of the seed laser oscillatorand amplifying the output of the seed laser oscillator to form a lasersystem output comprising a laser output light beam of pulses, the laseramplification stage comprising: a ring power amplification stagecomprising: a beam expander comprising a two prism beam expander; and apartially reflecting optical element through which the seed laseroscillator output light beam is injected into the ring poweramplification stage; and a coherence busting mechanism intermediate theseed laser oscillator and the ring power amplification stage comprisingan optical delay path having a delay length longer than the coherencelength of a pulse in the seed laser oscillator laser output light beamof pulses; wherein the first prism of the two prism beam expandercomprises a split prism and the second prism of the two prism beamexpander comprises a one piece prism.